The relaxation of phenylephrine-contracted blood vessels by acetylcholine, nitroprusside, or atrial natriuretic factor has been linked to elevations in cyclic guanosine 3',5'-monophosphate (cGMP). Also, 8-bromo-cGMP can induce vascular relaxation in isolated vascular smooth muscle contracted with phenylephrine. We determined whether these cGMP-dependent vasodilators could relax isolated rat aortas contracted with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. cGMP was measured by radioimmunoassay. Acetylcholine, nitroprusside, and atrial natriuretic factor induced relaxation in vascular smooth muscle contracted by 12-O-tetradecanoylphorbol-13-acetate. These relaxation responses were accompanied by elevations of cGMP. However, the sensitivity to these vasodilators was markedly decreased in phorbol ester-contracted vessels compared to phenylephrine-contracted vessels. Nifedipine and superoxide dismutase induced small but significant relaxations in phorbol ester-contracted vessels; however, blood vessels contracted with phenylephrine and phorbol ester relaxed completely with papaverine. There was a marked decrease in sensitivity to 8-bromo-cGMP in phorbol ester-treated vessels compared to phenylephrine-contracted vessels. Contractions induced by phorbol ester were not inhibited by amiloride or chlorpromazine. Also, following incubation in potassium-free salt solution, vessels incubated with phenylephrine or phenylephrine and phorbol ester underwent similar relaxations when exposed to potassium chloride. The contractile state induced by phorbol ester has decreased sensitivity to cGMP-dependent vasodilators. This may be due to nonspecific effects of the phorbol ester or to the mechanism by which protein kinase C activation maintains vascular tone.