diff --git a/src/CoreOperators.ts b/src/CoreOperators.ts
index dfe4614624..5ea980e2cf 100644
--- a/src/CoreOperators.ts
+++ b/src/CoreOperators.ts
@@ -56,7 +56,7 @@ export interface CoreOperators<T> {
   partition?: (predicate: (x: T) => boolean) => Observable<T>[];
   publish?: () => ConnectableObservable<T>;
   publishBehavior?: (value: any) => ConnectableObservable<T>;
-  publishReplay?: (bufferSize: number, windowTime: number, scheduler?: Scheduler) => ConnectableObservable<T>;
+  publishReplay?: (bufferSize?: number, windowTime?: number, scheduler?: Scheduler) => ConnectableObservable<T>;
   reduce?: <R>(project: (acc: R, x: T) => R, seed?: R) => Observable<R>;
   repeat?: (count?: number) => Observable<T>;
   retry?: (count?: number) => Observable<T>;
@@ -91,4 +91,4 @@ export interface CoreOperators<T> {
   withLatestFrom?: <R>(...observables: Array<Observable<any> | ((...values: Array<any>) => R)>) => Observable<R>;
   zip?: <R>(...observables: Array<Observable<any> | ((...values: Array<any>) => R)>) => Observable<R>;
   zipAll?: <R>(project?: (...values: Array<any>) => R) => Observable<R>;
-}
\ No newline at end of file
+}
diff --git a/src/Observable.ts b/src/Observable.ts
index d07061ecc3..20b3017573 100644
--- a/src/Observable.ts
+++ b/src/Observable.ts
@@ -210,7 +210,7 @@ export class Observable<T> implements CoreOperators<T>  {
   partition: (predicate: (x: T) => boolean) => Observable<T>[];
   publish: () => ConnectableObservable<T>;
   publishBehavior: (value: any) => ConnectableObservable<T>;
-  publishReplay: (bufferSize: number, windowTime: number, scheduler?: Scheduler) => ConnectableObservable<T>;
+  publishReplay: (bufferSize?: number, windowTime?: number, scheduler?: Scheduler) => ConnectableObservable<T>;
   reduce: <R>(project: (acc: R, x: T) => R, seed?: R) => Observable<R>;
   repeat: (count?: number) => Observable<T>;
   retry: (count?: number) => Observable<T>;