Release-regulating 5-hydroxytryptamine (5-HT) autoreceptors in the rat brain belong to the 5-HT1B subtype. On the other hand, the human brain seems to lack 5-HT1B receptors. In the present work 5-HT autoreceptors present in human brain were characterized pharmacologically. Synaptosomes prepared from biopsy samples of human neocortex were labeled with [3H]5-HT and exposed in superfusion to selective 5-HT receptor agonists and antagonists during K+ depolarization. The rank order of potency of agonists as inhibitors of the [3H]5-HT overflow was 5-HT > sumatriptan (5-HT1D/1B) > 8-hydroxy-2-(di-n-propylamino)tetralin (5-HT1A/1D) >> 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (5-HT2/1C). The effect of 5-HT was insensitive to ketanserin (5-HT2) but antagonized by methiothepin (5-HT1/2) or by metergoline (5-HT1C/1D). The data are compatible with a classification of the human 5-HT autoreceptor as being of the 5-HT1D subtype.