Following microinjection of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the medial prefrontal cortex (defined as the neocortical area innervated by the thalamic mediodorsal nucleus) labelled cells were observed in the pyramidal layer of the CA1 field of Ammon's horn. Observations made using antidromic stimulation confirmed these results, and revealed that slow conduction velocity of the fibers of the hippocampal cells innervating the prefrontal cortex. Taken together, these data provide evidence for a direct projection of CA1 cells to the medial prefrontal cortex.