The contractile effects of the phorbol-12,13-dibutyrate (PDBu) on rabbit basilar, renal, and saphenous arterial rings were investigated under various bathing conditions. At millimolar concentrations of Ca2+, the contractile responses with respect to both maximal contraction and sensitivity to PDBu were in the rank order basilar artery greater than renal artery greater than saphenous artery. Contractions were induced by PDBu also at micromolar concentrations of Ca2+. Depolarization of the cell membrane in the presence of micromolar Ca2+, which did not result in contraction of the vascular rings, caused a shift of the concentration response curves towards lower PDBu concentrations. These results are compatible with the assumption that PDBu induces contraction in vascular smooth muscle by activating protein kinase C.