5-hydroxytryptamine (5-HT) (3 X 10(-9) to 10(-6) M) produced a concentration-related inhibition of potassium-evoked tritium release from slices of rat hypothalamus preloaded with [3H]-5-HT. The response to 5-HT was unaffected by the presence of yohimbine (10(-6) M), pimozide (10(-7) M), domperidone (10(-7) M) or tetrodotoxin (10(-7) M), indicating that the response was not mediated via alpha 1- or alpha 2-adrenoceptors or dopamine receptors and that the receptors that were involved were located directly on the 5-HT nerve terminal. The 5-HT antagonist metergoline (10(-8) to 3 X 10(-7) M) produced a parallel rightward shift in the concentration-effect curve to 5-HT with no reduction in the size of the maximum response. The pA10 value for metergoline was 6.82 and the slope of the Arunlakshana-Schild plot was not significantly different from 1.0 indicating that it was a competitive antagonist. Methiothepin produced a similar effect to metergoline whilst cyproheptadine and methysergide were less potent as antagonists of 5-HT and were not competitive. Cinanserin was inactive. Thus we have characterized the 5-HT autoreceptor in the rat hypothalamus using a classical pharmacological approach and found that it has more in common with the autoreceptor which we have previously identified in the raphe nuclei of the rat than it has with the 5-HT receptor located on dopamine neuroterminals in the striatum.