The effects of melatonin on the contractile responses to 5-hydroxytryptamine (5-HT), norepinephrine (NE), angiotensin-1 (AT-1) and potassium were determined on the rabbit isolated aorta, iliac and renal arteries. Melatonin, at 10(-4) or 10(-3) M inhibited the response to 5-HT in the aorta, iliac and renal arteries. Melatonin, at 10(-3) M, had a negligible effect on the responses to NE and AT-1 in all preparations used. The potassium-induced contraction of all 3 preparations used was slightly inhibited by only the high concentration of melatonin (10(-3) M). In a Ca+-free medium with EGTA (0.1 mM), the residual response to 5-HT (10(-5) or 10(-4) M) were inhibited by melatonin (10(-4) or 10(-3) M) in all preparations used. In a Ca2+-free medium with EGTA (0.01 mM), nifedipine (10(-6) M) and 5-HT (5 X 10(-7) M), application of Ca2+ (2 mM) resulted in a tonic contraction, related to receptor operated channels, of all preparations. This Ca2+ dependent, nifedipine insensitive contraction was markedly inhibited or abolished by melatonin at 10(-5) and 10(-4) M. These results demonstrated that melatonin had a generally greater inhibitory effect on the response to 5-HT than those to NE, AT-1 and potassium in vascular smooth muscles. Also the results suggest that action of melatonin on the 5-HT induced contraction is more related to interference with Ca2+ influx through receptor operated channels than release of intracellular Ca2+ from the store site.