Phorbol esters, which activate protein kinase C, modulate vasoconstrictor-induced tension in vascular smooth muscle. We examined the effects of phorbol esters (phorbol 12,13-dibutyrate [PDBu] and 12-O-tetradecanoylphorbol 13-acetate [TPA]) on receptor agonist (serotonin [5-HT] and arginine vasopressin [AVP])-, high K+-, and caffeine-induced contractions in rings of rat aorta and a small (second-order) branch of the superior mesenteric artery (SMA). PDBu and TPA significantly augmented agonist-evoked contractions in aorta but diminished those in SMA. For example, 30 nM PDBu increased 5-HIT- and AVP-evoked contractions 2.0–2.5-fold in aorta (p<0.01) but decreased 5-HIT- and AVP-induced contractions by 40–60% in SMA (p<0.01). In contrast, PDBu and TPA amplified high K+- and 10 mM caffeine-induced contractions in both aorta and SMA. Augmentation of agonist-induced contractions by PDBu was greater in endothelium-denuded aorta than in intact aorta. Two protein kinase C antagonists, H-7 and staurosporine, inhibited 5-HT-evoked contractions in the absence as well as in the presence of PDBu in both types of arteries. The augmentation of contractile responses to caffeine and K+by phorbol esters in both types of arteries suggests that the phorbols increase the sensitivity of the contractile apparatus to Ca2+, probably by activating protein kinase C. However, the inhibitory effects of phorbols on 5-Hr- and AVP-evoked responses in SMA suggest that under these conditions the dominant effect of the phorbols is a marked reduction in the availability of Ca2+in the SMA but not in the aorta. (Circulation Research1992;70:978–990)