Hippocampal slices were prepared from brain tissue of patients undergoing neurosurgery for epilepsy. The slices were incubated with 3H-serotonin (3H-5-HT) and then superfused with physiological salt solution containing 6-nitroquipazine. Tritium overflow was evoked either electrically (3 Hz) or by K+ 25 mM. The electrically evoked overflow of tritium was almost abolished by tetrodotoxin or by omission of Ca2+ ions. 5-HT 0.1 and 1 microM reduced the evoked overflow by 38 and 55%, respectively. The effect of 5-HT 1 microM was abolished by the 5-HT1/2 receptor antagonist methiothepin 1 microM, which, by itself, increased the evoked overflow by 59%. Tritium overflow evoked by high K+ in slices superfused with medium containing tetrodotoxin was reduced by 5-HT 1 microM by 49%. This effect was markedly attenuated by methiothepin 0.32 microM, which, by itself, tended to increase the evoked overflow. The results show that the serotoninergic neurones of the human hippocampus are endowed with presynaptic inhibitory autoreceptors.