The hormone melatonin potentiated contractile responses to adrenergic nerve stimulation in isolated ring segments of rat caudal artery. This effect was inhibited by the melatonin receptor antagonist luzindole but not by the serotonin 5-HT2 receptor antagonist ketanserin. Melatonin had no direct effects on vascular tone. Melatonin agonists potentiated contractile responses with a relative order of potency (2-iodomelatonin, EC50 = 0.6 nM; melatonin, EC50 = 4.7 nM; N-acetylserotonin, EC50 = 1.5 microM) that is consistent with the melatonin ML1 receptor subtype. Melatonin also potentiated contractions elicited by exogenous norepinephrine and produced its effects in the absence of an intact endothelium. These data suggest that melatonin acts on receptors in the smooth muscle. The caudal artery provides a useful functional assay for pharmacological analysis of melatonin receptors. Physiologically, melatonin may activate its receptors at night to influence thermoregulation in the rat by enhancing the effects of sympathetic input to the caudal artery.