Serotonin (5-hydroxytryptamine, 5-HT) has been shown to inhibit the rhythmic constrictions, accompanied by an increase in cAMP synthesis, in porcine pial veins. Since porcine pial veins contain predominant postsynaptic alpha 2-adrenoceptors which are coupled to Gi-protein, the possibility that the inhibitory effect of 5-HT is antagonized by norepinephrine was examined pharmacologically, using tissue bath techniques. The results indicated that norepinephrine (0.1-1 microM) attenuated 5-HT-induced inhibition of rhythmic constriction. This effect of norepinephrine was mimicked by clonidine (an alpha 2-adrenoceptor agonist), but not by methoxamine (an alpha 1-adrenoceptor agonist). Furthermore, the effect of norepinephrine was prevented by yohimbine (an alpha 2-adrenoceptor antagonist) and pertussis toxin, but was not prevented by prazosin (an alpha 1-adrenoceptor antagonist). In parallel studies, the basal concentration of cAMP and that induced by 5-HT in the pial veins were inhibited by norepinephrine (0.3 microM). These results are consistent with the previous findings that 5-HT-induced inhibition of rhythmic constriction in the porcine pial veins is associated with an increase in vascular cAMP synthesis and suggest that norepinephrine attenuates 5-HT-induced inhibition of rhythmic constriction in part by negatively coupling to adenylate cyclase via alpha 2-adrenoceptors.