diff --git a/README.md b/README.md
index 84f8ef4..1086af0 100644
--- a/README.md
+++ b/README.md
@@ -138,6 +138,8 @@ type res5 = Pipe<
   - [x] `ReturnType<Fn>`
   - [x] `Parameters<Fn>`
   - [x] `Parameter<N, Fn>`
+  - [x] `MapReturnType<Fn, FunctionType>`
+  - [x] `MapParameters<Fn, FunctionType>`
 - [ ] Tuples
   - [x] `Create<X> -> [X]`
   - [x] `Partition<Tuple>`
diff --git a/src/internals/functions/Functions.ts b/src/internals/functions/Functions.ts
index dc8a5b2..b0bc6fe 100644
--- a/src/internals/functions/Functions.ts
+++ b/src/internals/functions/Functions.ts
@@ -1,4 +1,4 @@
-import { Fn, PartialApply, unset, _ } from "../core/Core";
+import { Fn, PartialApply, unset, _, Call } from "../core/Core";
 
 export namespace Functions {
   type ParametersImpl<fn> = fn extends (...args: infer args) => any
@@ -67,4 +67,62 @@ export namespace Functions {
   export interface ReturnTypeFn extends Fn {
     return: ReturnTypeImpl<this["arg0"]>;
   }
+
+  /**
+   * Transforms the return type of a function type.
+   *
+   * @param fn - Type-level function to call on the return type.
+   * @param fnValue - The function type to update.
+   * @returns a function type with an updated return type.
+   *
+   * @example
+   * ```ts
+   * type T0 = Call<
+   *   Functions.MapReturnType<Numbers.ToString>,
+   *   (a: number, b: string) => 1 | 2
+   * >;
+   * // => (a: number, b: string) => "1" | "2"
+   * ```
+   */
+  export type MapReturnType<
+    fn extends Fn | unset | _ = unset,
+    fnValue extends ((...args: any[]) => any) | _ | unset = unset
+  > = PartialApply<MapReturnTypeFn, [fn, fnValue]>;
+
+  export interface MapReturnTypeFn extends Fn {
+    return: this["args"] extends [infer fn extends Fn, infer fnValue]
+      ? fnValue extends (...args: infer args) => infer returnType
+        ? (...args: args) => Call<fn, returnType>
+        : never
+      : never;
+  }
+
+  /**
+   * Transforms the paramaters of a function type.
+   *
+   * @param fn - Type-level function to call on parameters.
+   * @param fnValue - The function type to update.
+   * @returns a function type with updated parameters.
+   *
+   * @example
+   * ```ts
+   * type T0 = Call<
+      F.MapParameters<Tuples.Map<Strings.ToNumber>>,
+      (a: "1" | "2", b: "3" | "4") => void
+    >;
+   * // => (a: 1 | 2, b: 3 | 4) => void
+   * ```
+   */
+  export type MapParameters<
+    fn extends Fn | unset | _ = unset,
+    fnValue extends ((...args: any[]) => any) | _ | unset = unset
+  > = PartialApply<MapParametersFn, [fn, fnValue]>;
+
+  export interface MapParametersFn extends Fn {
+    return: this["args"] extends [infer fn extends Fn, infer fnValue]
+      ? fnValue extends (...args: infer args) => infer returnType
+        ? (...args: Extract<Call<fn, args>, readonly any[]>) => returnType
+        : never
+      : never;
+  }
 }
diff --git a/test/functions.test.ts b/test/functions.test.ts
index 4a8edf2..793ec0f 100644
--- a/test/functions.test.ts
+++ b/test/functions.test.ts
@@ -1,4 +1,4 @@
-import { F, _ } from "../src/index";
+import { F, Numbers, Strings, Tuples, _ } from "../src/index";
 import { Call } from "../src/internals/core/Core";
 import { Equal, Expect } from "../src/internals/helpers";
 
@@ -23,4 +23,20 @@ describe("Functions", () => {
     //   ^?
     type tes1 = Expect<Equal<res1, boolean>>;
   });
+  it("MapReturnType", () => {
+    type res1 = Call<
+      //   ^?
+      F.MapReturnType<Strings.ToNumber>,
+      (a: string, b: number) => "1" | "2"
+    >;
+    type tes1 = Expect<Equal<res1, (a: string, b: number) => 1 | 2>>;
+  });
+  it("MapParameters", () => {
+    type res1 = Call<
+      //   ^?
+      F.MapParameters<Tuples.Map<Strings.ToNumber>>,
+      (a: "1" | "2", b: "3" | "4") => void
+    >;
+    type tes1 = Expect<Equal<res1, (a: 1 | 2, b: 3 | 4) => void>>;
+  });
 });