Based on the previous finding that 5-hydroxytryptamine (5-HT) co-exists with norepinephrine (NE) in cerebrovascular sympathetic nerve fibers and can be released during electrical nerve stimulation, the postjunctional interaction between the two amines was studied in isolated basilar artery of guinea pig. A low concentration of 5-HT, which in itself has little or no constrictive effect, potentiated the weak contraction of NE by almost 300%. The amplification was antagonized by methiothepin, but not by ketanserin, and it could be mimicked by methysergide. The marked potentiation is thus probably associated with the 5-HT1-like receptors, which earlier have been found to mediate the direct vasoconstrictive action of 5-HT in this vessel preparation.