diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..596f1d3
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1 @@
+pytest_plugins = ("tests.requests_mocker",)
diff --git a/tests/requests_mocker.py b/tests/requests_mocker.py
new file mode 100644
index 0000000..e045309
--- /dev/null
+++ b/tests/requests_mocker.py
@@ -0,0 +1,106 @@
+"""A mocker for ``urllib.request.urlopen``."""
+
+import collections
+import dataclasses
+import json as jsonlib
+import typing
+import unittest.mock
+import urllib.request
+
+import pytest
+
+
+@pytest.fixture(name="requests_mocker")
+def get_mock():
+    """Return an instance of ``Mock`` to be used as a fixture.
+
+    Example::
+
+        with get_mock() as mock:
+            mock.register("GET", "https://example.com", content="OK")
+            # call code that would make an HTTP request
+    """
+    m = Mock()
+    with unittest.mock.patch("urllib.request.urlopen", m.urlopen):
+        yield m
+
+
+@dataclasses.dataclass
+class Request:
+    url: str
+    headers: typing.Dict[str, str]
+    data: bytes
+    params: dict
+
+
+@dataclasses.dataclass
+class Response:
+    content: bytes
+    status: int
+
+    def read(self):
+        return self.content
+
+
+@dataclasses.dataclass
+class Call:
+    request: Request
+    response: Response
+
+
+class Mock():
+    """Intercept HTTP requests and mock their responses.
+
+    An instance of ``Mock`` can be configured via its two methods:
+
+    - ``get(url: str, json: object, status=200)`` allows you to mock
+      the response of a ``GET`` request to particular URL.
+
+    - ``register(method: str, url: str, content: bytes, status=200)``
+       is a more generic method.
+    """
+    def __init__(self):
+        self.mocks = collections.defaultdict(dict)
+        self.calls = []
+
+    def register(self, method: str, url: str, content: bytes, status: int = 200):
+        method = method.lower()
+        self.mocks[url][method] = Response(content=content, status=status)
+
+    def get(self, url: str, json: object, status: int = 200):
+        content = jsonlib.dumps(json)
+        self.register("get", url, content=content, status=status)
+
+    def urlopen(self, request: urllib.request.Request, **kwargs):
+        method = request.get_method().lower()
+        url = _strip_query_string(request.full_url)
+        response = self.mocks.get(url, {}).get(method)
+        if not response:
+            raise ValueError(f"No mock for method={method} and url={url}")
+        call = Call(
+            request=Request(
+                url=url,
+                headers=request.headers,  # type: ignore[arg-type]
+                data=request.data or b'',  # type: ignore[arg-type]
+                params=_extract_params(request),
+            ),
+            response=response,
+        )
+        self.calls.append(call)
+        return response
+
+
+def _strip_query_string(url: str) -> str:
+    parsed = urllib.parse.urlparse(url)
+    return parsed._replace(query="").geturl()
+
+
+def _extract_params(request: urllib.request.Request) -> dict:
+    query = urllib.parse.urlparse(request.full_url).query
+    if not query:
+        return {}
+    params = urllib.parse.parse_qs(query)
+    for key, values in params.items():
+        if len(values) == 1:
+            params[key] = values[0]
+    return params
diff --git a/tests/test_githost.py b/tests/test_githost.py
new file mode 100644
index 0000000..0f0b817
--- /dev/null
+++ b/tests/test_githost.py
@@ -0,0 +1,28 @@
+import os
+from unittest import mock
+
+from check_oldies import branches
+from check_oldies import githost
+
+
+FAKE_GITHUB_API_RESPONSE = [
+    {
+        "number": 1234,
+        "state": "open",
+        "html_url": "https://github.com/polyconseil/check-oldies/pull/1234",
+    },
+]
+
+
+@mock.patch.dict(os.environ, {"TOKEN": "secret"}, clear=True)
+def test_github_api(requests_mocker):
+    requests_mocker.get(
+        "https://api.github.com/repos/polyconseil/check-oldies/pulls",
+        json=FAKE_GITHUB_API_RESPONSE,
+    )
+    api_access = branches.GitHostApiAccessInfo(auth_token_env_var="TOKEN")
+    api = githost.GitHubApi("polyconseil", api_access)
+    pull_request = api.get_pull_request("check-oldies", "my-branch")
+    assert pull_request.number == 1234
+    assert pull_request.state == "open"
+    assert pull_request.url == "https://github.com/polyconseil/check-oldies/pull/1234"
diff --git a/tests/test_requests_mocker.py b/tests/test_requests_mocker.py
new file mode 100644
index 0000000..74197b3
--- /dev/null
+++ b/tests/test_requests_mocker.py
@@ -0,0 +1,16 @@
+import urllib.request
+
+from . import requests_mocker
+
+
+def test_extract_params():
+    url = "https://example.com/?single=1&multiple=2&multiple=3&empty="
+    request = urllib.request.Request(url)
+    params = requests_mocker._extract_params(request)
+    assert params == {"single": "1", "multiple": ["2", "3"]}
+
+
+def test_strip_query_string():
+    url = "https://example.com/path?foo=1"
+    stripped = requests_mocker._strip_query_string(url)
+    assert stripped == "https://example.com/path"