The mechanism of action of many direct-acting vascular smooth muscle relaxant agents is undefined. Moreover, an additional intracellular locus of action for some Ca++ entry blockers has been proposed. We have examined the potential for direct action by some of these agents at the level of smooth muscle contractile proteins by quantitating changes in Ca++-dependent superprecipitation of native arterial actomyosin. Moreover, inasmuch as previous research has linked phosphorylation of the 20,000 dalton myosin light chain to Ca++-dependent regulation of contraction, effects on myosin phosphorylation also were quantitated. Whereas the standard calmodulin antagonist W-7 inhibited both parameters by approximately 50% at 10(-4) M, diazoxide, hydralazine, 3-isobutyl-1-methylxanthine, papaverine, propranolol, nifedipine, nitrendipine, sodium nitroprusside and verapamil did not significantly inhibit either parameter at equimolar concentrations. Cyclandelate significantly delayed the onset of superprecipitation, but did not affect the extent of superprecipitation or myosin phosphorylation. The Ca++ antagonists felodipine and diltiazem inhibited superprecipitation by approximately 25%. However, unlike W-7, or felodipine, diltiazem did not concomitantly inhibit myosin phosphorylation. Inhibition of superprecipitation by diltiazem was apparent at a concentration of 10(-6) M, was manifest by a rightward shift in the pCa relationship and could be attenuated by exogenous calmodulin. These results show that most vasodilators do not have direct effects on smooth muscle contractile protein function. However, diltiazem may inhibit Ca++-dependent arterial actin-myosin interactions by a mechanism which is independent of regulation of myosin light chain phosphorylation.