The cortical projection area of the thalamic mediodorsal nucleus (MD) was investigated using the horseradish peroxidase technique. Distinct relations between subdivisions of MD and of the prefrontal cortex were established: the medial sector of MD projects to sulcal cortex dorsal to the rhinal fissure and to the lateral frontopolar region; the lateral sector of MD reaches mainly cortex within the medial part of the hemisphere from the frontopolar region up to a mid corpus callosal level. The projection area of the lateral sector of MD overlaps with the projection fields of the paratenial and anteromedial nuclei. It is concluded that the guinea pig''s prefrontal cortex, defined as the projection area of MD, has strong similarities with the prefrontal cortices of rats and rabbits, that is with the prefrontal cortices of species of two different orders.