5-Hydroxytryptophol (5-HTOL) caused a dose-dependent contraction of helically-cut strips of dog cerebral arteries. The 5-HTOL-induced contraction was suppressed by cinanserin, as was the contraction induced by 5-hydroxytryptamine (5-HT). Treatment with 5-HTOL shifted the dose-response curve for 5-HT to the right and downward in a dose-dependent manner, but did not attenuate the contractile response to prostaglandin F2a. It may be concluded that 5-HTOL elicits cerebroarterial contractions by activating tryptaminergic receptors and also interferes with the action of 5-HT on the receptors.