diff --git a/_unittests/ut_gdot/test_gdot_extension.py b/_unittests/ut_gdot/test_gdot_extension.py
index d1ee7a3..dea0ff4 100644
--- a/_unittests/ut_gdot/test_gdot_extension.py
+++ b/_unittests/ut_gdot/test_gdot_extension.py
@@ -1,6 +1,8 @@
 import unittest
 import logging
+import os
 import sys
+from contextlib import contextmanager
 from sphinx_runpython.process_rst import rst2html
 from sphinx_runpython.ext_test_case import (
     ExtTestCase,
@@ -10,6 +12,20 @@
 )
 
 
+@contextmanager
+def unittest_going():
+    """Context manager that sets UNITTEST_GOING=1 for the duration of the block."""
+    old = os.environ.get("UNITTEST_GOING", None)
+    os.environ["UNITTEST_GOING"] = "1"
+    try:
+        yield
+    finally:
+        if old is None:
+            os.environ.pop("UNITTEST_GOING", None)
+        else:
+            os.environ["UNITTEST_GOING"] = old
+
+
 class TestGDotExtension(ExtTestCase):
     def setUp(self):
         logger = logging.getLogger("gdot")
@@ -146,6 +162,59 @@ def test_gdot4_png(self):
             return
         self.assertIn("png", content)
 
+    @ignore_warnings(PendingDeprecationWarning)
+    def test_gdot_unittest_going_svg(self):
+        """When UNITTEST_GOING=1, a dummy SVG containing 'DISABLED FOR TESTS' is rendered."""
+        content = """
+                    before
+
+                    .. gdot::
+                        :format: svg
+
+                        digraph foo {
+                          "bar" -> "baz";
+                        }
+
+                    after
+                    """.replace("                    ", "")
+
+        with unittest_going():
+            html = rst2html(
+                content, writer_name="html", new_extensions=["sphinx_runpython.gdot"]
+            )
+
+        self.assertIn("DISABLED FOR TESTS", html)
+        self.assertIn("<svg", html)
+        self.assertNotIn("<img", html)
+
+    @ignore_warnings(PendingDeprecationWarning)
+    def test_gdot_unittest_going_png(self):
+        """
+        When UNITTEST_GOING=1, a dummy image containing
+        'DISABLED FOR TESTS' is rendered.
+        """
+        content = """
+                    before
+
+                    .. gdot::
+                        :format: png
+
+                        digraph foo {
+                          "bar" -> "baz";
+                        }
+
+                    after
+                    """.replace("                    ", "")
+
+        with unittest_going():
+            html = rst2html(
+                content, writer_name="html", new_extensions=["sphinx_runpython.gdot"]
+            )
+
+        self.assertIn("DISABLED FOR TESTS", html)
+        self.assertIn("<img", html)
+        self.assertNotIn("<svg", html)
+
 
 if __name__ == "__main__":
     unittest.main(verbosity=2)
diff --git a/sphinx_runpython/gdot/sphinx_gdot_extension.py b/sphinx_runpython/gdot/sphinx_gdot_extension.py
index 05b7eca..da1905b 100644
--- a/sphinx_runpython/gdot/sphinx_gdot_extension.py
+++ b/sphinx_runpython/gdot/sphinx_gdot_extension.py
@@ -216,6 +216,30 @@ def depart_gdot_node_rst(self, node):
     self.end_state(wrap=False)
 
 
+_DUMMY_SVG = (
+    '<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">'
+    '<text x="10" y="50">DISABLED FOR TESTS</text>'
+    "</svg>"
+)
+
+_DUMMY_PNG_HTML = (
+    '<img src="data:image/png;base64,'
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAC0lEQVQI12NgAAIABQ"
+    'AABjkB6QAAAABJRU5ErkJggg==" alt="DISABLED FOR TESTS" />'
+)
+
+
+def _emit_dummy_output(self, format: str = "svg"):
+    """Emit a placeholder graphic when ``UNITTEST_GOING=1`` is set."""
+    self.body.append('<div class="graphviz">')
+    if format == "png":
+        self.body.append(_DUMMY_PNG_HTML)
+    else:
+        self.body.append(_DUMMY_SVG)
+    self.body.append("</div>\n")
+    raise nodes.SkipNode
+
+
 def render_dot_html(
     self,
     node: gdot_node,
@@ -227,6 +251,9 @@ def render_dot_html(
     filename: str | None = None,
     format: str = "svg",
 ) -> tuple[str, str]:
+    if os.environ.get("UNITTEST_GOING", "0") == "1":
+        _emit_dummy_output(self, format=format)
+
     if format not in {"png", "svg"}:
         logger = logging.getLogger(__name__)
         logger.warning(__("format must be either 'png' or 'svg', but is %r"), format)
@@ -362,6 +389,8 @@ def visit_gdot_node_html(self, node):
     and the :epkg:`SVG` format.
     """
     if node["format"].lower() == "png":
+        if os.environ.get("UNITTEST_GOING", "0") == "1":
+            _emit_dummy_output(self, format="png")
         from sphinx.ext.graphviz import html_visit_graphviz
 
         return html_visit_graphviz(self, node)