The present in vivo studies were undertaken to evaluate electrophysiologically the modulatory role of the terminal 5-HT autoreceptor on 5-HT neurotransmission. In a first series of experiments, the effect of the electrical activation of the ascending 5-HT pathway on the firing activity of CA3 hippocampal pyramidal neurons was measured before and after the intravenous administration of methiothepin, a terminal 5-HT autoreceptor antagonist. Methiothepin significantly increased the duration of the suppression of firing activity of these neurons by the electrical stimulation of the 5-HT pathway, without modifying their responsiveness to microiontophoretically applied 5-HT. This suggests that endogenously released 5-HT activates the 5-HT terminal autoreceptor and that methiothepin enhances the efficacy of 5-HT synaptic transmission by blocking this activation. In a second series of experiments, further evidence for the activation of terminal 5-HT autoreceptors by 5-HT released by the electrical stimulation was sought by assessing the effectiveness of 2 series of stimulations of the ascending 5-HT pathway delivered at different frequencies while recording the same postsynaptic neuron. Increasing the frequency of stimulation (from 0.8 to 5 Hz) significantly reduced the duration of suppression of firing activity of the postsynaptic neurons. This difference between the 0.8 and 5 Hz stimulations was decreased by intravenous methiothepin, suggesting that the reduced effectiveness of the stimulations delivered at the higher frequency is attributable to a greater activation of the terminal 5-HT autoreceptor. These results provide direct electrophysiological evidence for the modulatory role of the 5-HT terminal autoreceptor on 5-HT neurotransmission.