The effects of papaverine on cyclic[c]AMP, 45Ca uptake and force of contraction were studied in electrically stimulated left guinea-pig auricles. Papaverine (2 .times. 10-6-2 .times. 10-5 M) produced a concentration-dependent positive inotropic effect which was not affected by 10-7 M propranolol. Papaverine (10-5 M) increased cAMP; this effect developed faster than the increase in twitch tension. Papaverine (10-5 M) enhanced 45Ca uptake. The idea was supported of a causative series of interactions between a papaverine induced increase in cAMP, Ca uptake and force of contraction.