diff --git a/docs/packaging.rst b/docs/packaging.rst
index 77663ca..a86b417 100644
--- a/docs/packaging.rst
+++ b/docs/packaging.rst
@@ -50,6 +50,11 @@ You also have to have vdirsyncer fully installed at this point. Merely
 
 Running the tests happens with ``make test``.
 
+Hypothesis will randomly generate test input. If you care about deterministic
+tests, set the ``DETERMINISTIC_TESTS`` variable to ``"true"``::
+
+    make DETERMINISTIC_TESTS=true test
+
 Documentation
 =============