Experiments were carried out in vitro on a new antispasmodic drug, 2-(diethylamino)-1-methylethyl-cis-1-hydroxy[bicyclohexyl] -2-carboxylate (rociverine), to elucidate further the mechanism of its smooth-muscle relaxation. On the furtrethonium contractions of rat jejunum rociverine exerts both a competitive antagonism, about 3000 times less than that of atropine, and a noncompetitive antagonism equal to that of papaverine. Both on rat vas deferens and on rabbit aorta rociverine at low doses slightly potentiates the response to norepinephrine (unrelated to any inhibition of uptakes), whereas at higher doses it depresses the maximum effect, 10 times more on the vas deferens than on the aorta. Against calcium in the same preparations rociverine behaves as a competitive antagonist with 10 times less potency than verapamil on the vas deferens and 300 times less on the aorta. In negative inotropic and chronotropic action on guinea-pig atria rociverine is 300 and 100 times, respectively, less potent than verapamil and in negative inotropic action on KCl-depolarized guinea-pig ventricular strips, whose contractility is restored by histamine, it is 70 times less potent than verapamil. In sum, the smooth-muscle relaxant action of rociverine depends on a modest antimuscarinic action and a direct myolytic action, the latter probably being due to an inhibitory action on the transmembrane fluxes of Ca2+. The potency ratio relative to verapamil indicates that the anti-Ca2+ action of rociverine is greater on the visceral smooth muscle than on cardiac and vascular muscle.