BAY K 8644 augmented Ca2+-dependent action potentials and evoked a positive inotropic effect in embryonic chick ventricle. These effects are consistent with the properties of this Ca channel "agonist" whose actions are independent of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent phosphorylation. At low to intermediate concentrations (10(-8) to 10(-6) M), the cholinergic agonists acetylcholine, carbachol, and oxotremorine inhibit Ca2+-dependent action potentials and contractions in embryonic ventricle only in the presence of drugs that cause cAMP accumulation. The prediction that low to intermediate concentrations of these cholinergic agonists should not inhibit the effects of BAY K 8644 on Ca2+-dependent action potentials and contractions was borne out experimentally. This result is consistent with the cAMP hypothesis for muscarinic inhibition. It is noteworthy that all cholinergic agonists tested evoked a positive inotropic effect at high concentrations (greater than 10(-6) M) in the presence or absence of BAY K 8644. The positive inotropic effect was initiated by muscarinic receptors for it was blocked by atropine and was independent of endogenous catecholamines, since it occurred in the presence of propranolol. It is speculated that the positive inotropic effect of muscarinic agonists in embryonic heart muscle is related to stimulation of phosphoinositide metabolism.