The pineal hormone melatonin was found to decrease luminal diameter of rat middle cerebral artery segments, pressurized in vitro, in a concentration-dependent manner (concentration that produced a half-maximal effect = 2.7 nM). Contractile responses to melatonin were inhibited by luzindole, a melatonin receptor antagonist, but not by the serotonin receptor antagonist ketanserin. Pertussis toxin abolished the effect of melatonin, which is consistent with involvement of Gi or G(o) protein-coupled receptors. The maximal effect of melatonin was increased by elevating transmural pressure. When compared at the same pressure, contractions elicited by melatonin were smaller than those elicited by serotonin but similar in magnitude to those produced by tetraethylammonium or charybdotoxin, blockers of Ca(2+)-dependent, large-conductance K+ (BKCa) channels. The effect of melatonin was significantly attenuated in the presence of BKCa channel blockers, but not by apamin, a blocker of Ca(2+)-dependent, small-conductance K+ channels. Melatonin, like tetraethylammonium, significantly reduced vasodilation produced by NS-1619, an opener of BKCa channels. Contractile responses to melatonin were diminished in the presence of elevated extracellular K+ (16 mM), but they were not significantly affected by NG-nitro-L-arginine methyl ester. The results suggest that activation of melatonin receptors on rat cerebral arteries increases vascular tone through Gi or G(o) protein-mediated inhibition of BKCa channels. Thus melatonin, which is secreted during the night, can directly influence the contractile state of cerebral arteries.