diff --git a/packages/grpc-js-core/src/metadata.ts b/packages/grpc-js-core/src/metadata.ts
index 05c1e4ec..df3203fb 100644
--- a/packages/grpc-js-core/src/metadata.ts
+++ b/packages/grpc-js-core/src/metadata.ts
@@ -4,19 +4,21 @@ const LEGAL_KEY_REGEX = /^[0-9a-z_.-]+$/;
 const LEGAL_NON_BINARY_VALUE_REGEX = /^[ -~]*$/;
 
 export type MetadataValue = string|Buffer;
-
-export interface MetadataObject { [key: string]: MetadataValue[]; }
+export type MetadataObject = Map<string, MetadataValue[]>;
 
 function cloneMetadataObject(repr: MetadataObject): MetadataObject {
-  const result: MetadataObject = {};
-  forOwn(repr, (value, key) => {
-    result[key] = value.map(v => {
+  const result: MetadataObject = new Map<string, MetadataValue[]>();
+
+  repr.forEach((value, key) => {
+    const clonedValue: MetadataValue[] = value.map(v => {
       if (v instanceof Buffer) {
         return Buffer.from(v);
       } else {
         return v;
       }
     });
+
+    result.set(key, clonedValue);
   });
   return result;
 }
@@ -64,7 +66,7 @@ function validate(key: string, value?: MetadataValue): void {
  * A class for storing metadata. Keys are normalized to lowercase ASCII.
  */
 export class Metadata {
-  protected internalRepr: MetadataObject = {};
+  protected internalRepr: MetadataObject = new Map<string, MetadataValue[]>();
 
   /**
    * Sets the given value for the given key by replacing any other values
@@ -76,7 +78,7 @@ export class Metadata {
   set(key: string, value: MetadataValue): void {
     key = normalizeKey(key);
     validate(key, value);
-    this.internalRepr[key] = [value];
+    this.internalRepr.set(key, [value]);
   }
 
   /**
@@ -89,10 +91,13 @@ export class Metadata {
   add(key: string, value: MetadataValue): void {
     key = normalizeKey(key);
     validate(key, value);
-    if (!this.internalRepr[key]) {
-      this.internalRepr[key] = [value];
+
+    const existingValue: MetadataValue[]|undefined = this.internalRepr.get(key);
+
+    if (existingValue === undefined) {
+      this.internalRepr.set(key, [value]);
     } else {
-      this.internalRepr[key].push(value);
+      existingValue.push(value);
     }
   }
 
@@ -103,9 +108,7 @@ export class Metadata {
   remove(key: string): void {
     key = normalizeKey(key);
     validate(key);
-    if (Object.prototype.hasOwnProperty.call(this.internalRepr, key)) {
-      delete this.internalRepr[key];
-    }
+    this.internalRepr.delete(key);
   }
 
   /**
@@ -116,11 +119,7 @@ export class Metadata {
   get(key: string): MetadataValue[] {
     key = normalizeKey(key);
     validate(key);
-    if (Object.prototype.hasOwnProperty.call(this.internalRepr, key)) {
-      return this.internalRepr[key];
-    } else {
-      return [];
-    }
+    return this.internalRepr.get(key) || [];
   }
 
   /**
@@ -130,7 +129,8 @@ export class Metadata {
    */
   getMap(): {[key: string]: MetadataValue} {
     const result: {[key: string]: MetadataValue} = {};
-    forOwn(this.internalRepr, (values, key) => {
+
+    this.internalRepr.forEach((values, key) => {
       if (values.length > 0) {
         const v = values[0];
         result[key] = v instanceof Buffer ? v.slice() : v;
@@ -157,8 +157,11 @@ export class Metadata {
    * @param other A Metadata object.
    */
   merge(other: Metadata): void {
-    forOwn(other.internalRepr, (values, key) => {
-      this.internalRepr[key] = (this.internalRepr[key] || []).concat(values);
+    other.internalRepr.forEach((values, key) => {
+      const mergedValue: MetadataValue[] =
+          (this.internalRepr.get(key) || []).concat(values);
+
+      this.internalRepr.set(key, mergedValue);
     });
   }
 
@@ -168,7 +171,7 @@ export class Metadata {
   toHttp2Headers(): http2.OutgoingHttpHeaders {
     // NOTE: Node <8.9 formats http2 headers incorrectly.
     const result: http2.OutgoingHttpHeaders = {};
-    forOwn(this.internalRepr, (values, key) => {
+    this.internalRepr.forEach((values, key) => {
       // We assume that the user's interaction with this object is limited to
       // through its public API (i.e. keys and values are already validated).
       result[key] = values.map((value) => {
diff --git a/packages/grpc-js-core/test/test-metadata.ts b/packages/grpc-js-core/test/test-metadata.ts
index d60bd214..f5e4d80c 100644
--- a/packages/grpc-js-core/test/test-metadata.ts
+++ b/packages/grpc-js-core/test/test-metadata.ts
@@ -1,7 +1,7 @@
 import * as assert from 'assert';
 import * as http2 from 'http2';
 import {range} from 'lodash';
-import {Metadata} from '../src/metadata';
+import {Metadata, MetadataObject, MetadataValue} from '../src/metadata';
 
 class TestMetadata extends Metadata {
   getInternalRepresentation() {
@@ -277,21 +277,24 @@ describe('Metadata', () => {
       };
       const metadataFromHeaders = TestMetadata.fromHttp2Headers(headers);
       const internalRepr = metadataFromHeaders.getInternalRepresentation();
-      assert.deepEqual(internalRepr, {
-        key1: ['value1'],
-        key2: ['value2'],
-        key3: ['value3a', 'value3b'],
-        'key-bin': [
-          Buffer.from(range(0, 16)), Buffer.from(range(16, 32)),
-          Buffer.from(range(0, 32))
+      const expected: MetadataObject = new Map<string, MetadataValue[]>([
+        ['key1', ['value1']], ['key2', ['value2']],
+        ['key3', ['value3a', 'value3b']],
+        [
+          'key-bin',
+          [
+            Buffer.from(range(0, 16)), Buffer.from(range(16, 32)),
+            Buffer.from(range(0, 32))
+          ]
         ]
-      });
+      ]);
+      assert.deepEqual(internalRepr, expected);
     });
 
     it('creates an empty Metadata object from empty headers', () => {
       const metadataFromHeaders = TestMetadata.fromHttp2Headers({});
       const internalRepr = metadataFromHeaders.getInternalRepresentation();
-      assert.deepEqual(internalRepr, {});
+      assert.deepEqual(internalRepr, new Map<string, MetadataValue[]>());
     });
   });
 });