diff --git a/src/cli.ts b/src/cli.ts
index 71cb1ee..bfcf77d 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -123,6 +123,7 @@ function transformTypeScriptSource(source: string) {
   source = source.replace(/^import.*?$\n?/gm, '');
   // Add our imports
   source = `import { Observable } from 'rxjs';\n${source}`;
+  source = `import * as grpc from 'grpc';\n${source}`;
 
   if (source.includes('$protobuf')) {
     source = `import * as $protobuf from 'protobufjs';\n${source}`;
@@ -242,10 +243,10 @@ function cleanMethodSignatures(ast: Collection<ASTNode>) {
 
   // The promise variant of service methods are after the method declerations,
   // so the last method will have its comment followed by a return statement.
-  ast.find(jscodeshift.ExpressionStatement).forEach(fixReturnType);
-  ast.find(jscodeshift.ReturnStatement).forEach(fixReturnType);
+  ast.find(jscodeshift.ExpressionStatement).forEach(fixPromiseMethodSignature);
+  ast.find(jscodeshift.ReturnStatement).forEach(fixPromiseMethodSignature);
 
-  function fixReturnType(
+  function fixPromiseMethodSignature(
     path: ASTPath<jscodeshift.ExpressionStatement | jscodeshift.ReturnStatement>,
   ) {
     if (!path.node.comments) {
@@ -258,6 +259,11 @@ function cleanMethodSignatures(ast: Collection<ASTNode>) {
         changed = true;
         comment.value = comment.value.replace(returnsPromiseRe, '$1Observable$2');
         comment.value = comment.value.replace(/(@param\s+\{.*?\.)I([^.]+\})/g, '$1$2');
+        // Add optional metadata parameter
+        comment.value = comment.value.replace(
+          /^([\s\*]+@param\s+)(\{.*)$/gm,
+          '$1$2$1{grpc.Metadata=} metadata Optional metadata',
+        );
       }
     });
     if (changed) {
diff --git a/src/tests/metadata.test.ts b/src/tests/metadata.test.ts
new file mode 100644
index 0000000..b6fd49b
--- /dev/null
+++ b/src/tests/metadata.test.ts
@@ -0,0 +1,93 @@
+import * as cli from '../cli';
+
+import { compileInMemory } from './utils';
+
+describe('metadata test', () => {
+  let namespaces: string;
+
+  beforeAll(async () => {
+    namespaces = await cli.buildTypeScriptFromSources([
+      `
+        syntax = "proto3";
+        package test;
+
+        service Service {
+          rpc Single (Message) returns (Message) {}
+        }
+
+        message Message {
+          string field = 1;
+        }
+      `,
+    ]);
+  });
+
+  it('should accept metadata object', async () => {
+    const result = compileInMemory({
+      'grpc-namespaces.ts': namespaces,
+      'test.ts': `
+        import { Metadata } from 'grpc';
+        import { clientFactory } from 'rxjs-grpc';
+
+        import { test } from './grpc-namespaces';
+
+        const Services = clientFactory<test.ClientFactory>('test.proto', 'test');
+
+        const services = new Services('localhost:1234');
+        const Service = services.getService();
+
+        Service.single({ field: 'string' }, new Metadata());
+      `,
+    });
+    expect(result.errors).toEqual([]);
+    expect(result.ok).toBe(true);
+  });
+
+  it('should not accept invalid metadata object', async () => {
+    const result = compileInMemory({
+      'grpc-namespaces.ts': namespaces,
+      'test.ts': `
+        import { Metadata } from 'grpc';
+        import { clientFactory } from 'rxjs-grpc';
+
+        import { test } from './grpc-namespaces';
+
+        const Services = clientFactory<test.ClientFactory>('test.proto', 'test');
+
+        const services = new Services('localhost:1234');
+        const Service = services.getService();
+
+        Service.single({ field: 'string' }, { NOT_A_METADATA: 'yes' });
+      `,
+    });
+    expect(result.errors.length).toBeGreaterThan(0);
+    expect(result.ok).toBe(false);
+  });
+
+  it('should pass metadata object', async () => {
+    const result = compileInMemory({
+      'grpc-namespaces.ts': namespaces,
+      'test.ts': `
+        import { of } from 'rxjs';
+        import { serverBuilder } from 'rxjs-grpc';
+
+        import { test } from './grpc-namespaces';
+
+        const server = serverBuilder<test.ServerBuilder>('test.proto', 'test');
+
+        server.addService({
+
+          single(request, metadata) {
+            console.log(metadata.get('test'));
+            return of();
+          }
+
+        });
+
+        server.start('0.0.0.0:1234');
+      `,
+    });
+    expect(result.errors).toEqual([]);
+    expect(result.ok).toBe(true);
+  });
+});