Using an in vitro perfusion method, we examined the effects of cromakalim, a potassium channel opener, on the superior cerebellar arteries of 24 rats. Cromakalim had no effect on contractions induced by 129 mM K+ until a concentration of 10(-5) M was reached. Contractions evoked by 10(-5) M serotonin were attenuated by cromakalim in a concentration-dependent manner (p less than 0.05). The diameter of untreated superior cerebellar arteries remained almost constant with increasing perfusion pressure. However, in the presence of cromakalim vessel diameter increased with increasing perfusion pressure. At concentrations of 3 x 10(-6) M, cromakalim also inhibited basal myogenic tone and dilated unstimulated arteries (p less than 0.01). These results suggest that cromakalim is a cerebrovascular dilator acting on both receptor-mediated and myogenic contractions.