diff --git a/packages/metro/src/ModuleGraph/__tests__/ModuleGraph-test.js b/packages/metro/src/ModuleGraph/__tests__/ModuleGraph-test.js
index 06621fb5..26b048c3 100644
--- a/packages/metro/src/ModuleGraph/__tests__/ModuleGraph-test.js
+++ b/packages/metro/src/ModuleGraph/__tests__/ModuleGraph-test.js
@@ -30,28 +30,13 @@ describe('build setup', () => {
     const result = await buildSetup(noEntryPoints, noOptions);
 
     const [prelude] = result.modules;
-    expect(prelude).toEqual({
-      dependencies: [],
-      file: {
-        code:
-          'var __DEV__=true,__BUNDLE_START_TIME__=' +
-          'this.nativePerformanceNow?nativePerformanceNow():Date.now(),' +
-          "process={env:{NODE_ENV:'development'}};",
-        map: null,
-        path: '',
-        type: 'script',
-      },
-    });
+    expect(prelude).toMatchSnapshot();
   });
 
   it('sets `__DEV__` to false in the prelude if optimization is enabled', async () => {
     const result = await buildSetup(noEntryPoints, {optimize: true});
     const [prelude] = result.modules;
-    expect(prelude.file.code).toEqual(
-      'var __DEV__=false,__BUNDLE_START_TIME__=' +
-        'this.nativePerformanceNow?nativePerformanceNow():Date.now(),' +
-        "process={env:{NODE_ENV:'production'}};",
-    );
+    expect(prelude).toMatchSnapshot();
   });
 
   it('places the module system implementation directly after the prelude', async () => {
diff --git a/packages/metro/src/ModuleGraph/__tests__/__snapshots__/ModuleGraph-test.js.snap b/packages/metro/src/ModuleGraph/__tests__/__snapshots__/ModuleGraph-test.js.snap
new file mode 100644
index 00000000..8796999b
--- /dev/null
+++ b/packages/metro/src/ModuleGraph/__tests__/__snapshots__/ModuleGraph-test.js.snap
@@ -0,0 +1,25 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`build setup adds a prelude containing start time and \`__DEV__\` to the build 1`] = `
+Object {
+  "dependencies": Array [],
+  "file": Object {
+    "code": "var __DEV__=true,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process=this.process||{};process.env=process.env||{};process.env.NODE_ENV='development';",
+    "map": null,
+    "path": "",
+    "type": "script",
+  },
+}
+`;
+
+exports[`build setup sets \`__DEV__\` to false in the prelude if optimization is enabled 1`] = `
+Object {
+  "dependencies": Array [],
+  "file": Object {
+    "code": "var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process=this.process||{};process.env=process.env||{};process.env.NODE_ENV='production';",
+    "map": null,
+    "path": "",
+    "type": "script",
+  },
+}
+`;
diff --git a/packages/metro/src/integration_tests/__tests__/__snapshots__/basic_bundle-test.js.snap b/packages/metro/src/integration_tests/__tests__/__snapshots__/basic_bundle-test.js.snap
index 40681153..42f96672 100644
--- a/packages/metro/src/integration_tests/__tests__/__snapshots__/basic_bundle-test.js.snap
+++ b/packages/metro/src/integration_tests/__tests__/__snapshots__/basic_bundle-test.js.snap
@@ -1,7 +1,7 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`basic_bundle bundles package with polyfills 1`] = `
-"var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process={env:{NODE_ENV:'production'}};
+"var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process=this.process||{};process.env=process.env||{};process.env.NODE_ENV='production';
 (function (global) {
   'use strict';
 
@@ -189,7 +189,7 @@ require(4);"
 `;
 
 exports[`basic_bundle bundles package without polyfills 1`] = `
-"var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process={env:{NODE_ENV:'production'}};
+"var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process=this.process||{};process.env=process.env||{};process.env.NODE_ENV='production';
 (function (global) {
   'use strict';
 
diff --git a/packages/metro/src/lib/__tests__/getPreludeCode-test.js b/packages/metro/src/lib/__tests__/getPreludeCode-test.js
new file mode 100644
index 00000000..4773fb18
--- /dev/null
+++ b/packages/metro/src/lib/__tests__/getPreludeCode-test.js
@@ -0,0 +1,39 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @emails oncall+javascript_foundation
+ * @flow
+ * @format
+ */
+
+'use strict';
+
+const getPreludeCode = require('../getPreludeCode');
+const vm = require('vm');
+
+['development', 'production'].forEach(mode => {
+  describe(`${mode} mode`, () => {
+    it('sets up `process.env.NODE_ENV` and `__DEV__`', () => {
+      const sandbox: $FlowFixMe = {};
+      vm.createContext(sandbox);
+      vm.runInContext(getPreludeCode({isDev: mode == 'development'}), sandbox);
+      expect(sandbox.process.env.NODE_ENV).toEqual(mode);
+      expect(sandbox.__DEV__).toEqual(mode == 'development');
+    });
+
+    it('does not override an existing `process.env`', () => {
+      const nextTick = () => {};
+      const sandbox: $FlowFixMe = {process: {nextTick, env: {FOOBAR: 123}}};
+      vm.createContext(sandbox);
+      vm.runInContext(getPreludeCode({isDev: mode == 'development'}), sandbox);
+      expect(sandbox.process.env.NODE_ENV).toEqual(mode);
+      expect(sandbox.process.env.FOOBAR).toEqual(123);
+      expect(sandbox.process.nextTick).toEqual(nextTick);
+    });
+  });
+});
diff --git a/packages/metro/src/lib/getPreludeCode.js b/packages/metro/src/lib/getPreludeCode.js
index 1807010f..acde8c7e 100644
--- a/packages/metro/src/lib/getPreludeCode.js
+++ b/packages/metro/src/lib/getPreludeCode.js
@@ -16,7 +16,8 @@ function getPreludeCode({isDev}: {|+isDev: boolean|}): string {
   return (
     `var __DEV__=${String(isDev)},` +
     '__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),' +
-    `process={env:{NODE_ENV:'${isDev ? 'development' : 'production'}'}};`
+    'process=this.process||{};process.env=process.env||{};' +
+    `process.env.NODE_ENV='${isDev ? 'development' : 'production'}';`
   );
 }