diff --git a/httpx/_client.py b/httpx/_client.py
index 553ac7609a..21f3715f8e 100644
--- a/httpx/_client.py
+++ b/httpx/_client.py
@@ -849,13 +849,15 @@ def _send_single_request(self, request: Request, timeout: Timeout) -> Response:
         timer = Timer()
         timer.sync_start()
 
+        ext = request.ext.copy()
+        ext["timeout"] = timeout.as_dict()
         with map_exceptions(HTTPCORE_EXC_MAP, request=request):
             (status_code, headers, stream, ext) = transport.request(
                 request.method.encode(),
                 request.url.raw,
                 headers=request.headers.raw,
                 stream=request.stream,  # type: ignore
-                ext={"timeout": timeout.as_dict()},
+                ext=ext,
             )
 
         def on_close(response: Response) -> None:
@@ -1494,13 +1496,15 @@ async def _send_single_request(
         timer = Timer()
         await timer.async_start()
 
+        ext = request.ext.copy()
+        ext["timeout"] = timeout.as_dict()
         with map_exceptions(HTTPCORE_EXC_MAP, request=request):
             (status_code, headers, stream, ext,) = await transport.arequest(
                 request.method.encode(),
                 request.url.raw,
                 headers=request.headers.raw,
                 stream=request.stream,  # type: ignore
-                ext={"timeout": timeout.as_dict()},
+                ext=ext,
             )
 
         async def on_close(response: Response) -> None:
diff --git a/httpx/_models.py b/httpx/_models.py
index 4ef35ed88a..ae0d6680b5 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -760,6 +760,7 @@ def __init__(
         files: RequestFiles = None,
         json: typing.Any = None,
         stream: ByteStream = None,
+        ext: dict = None,
     ):
         if isinstance(method, bytes):
             self.method = method.decode("ascii").upper()
@@ -769,6 +770,7 @@ def __init__(
         self.headers = Headers(headers)
         if cookies:
             Cookies(cookies).set_cookie_header(self)
+        self.ext = {} if ext is None else ext
 
         if stream is not None:
             # There's an important distinction between `Request(content=...)`,