Abstract The experiments performed in this study were designed to test the hypothesis that cyclic nucleotides mediate the effects of adrenergic and cholinergic neurotransmitters on frog ventricle. A physiological approach was to measure the twitch tension and contracture tension of ventricular strips in the presence of exogenous neurotransmitter agonists. These observations were correlated to biochemical measurements of cyclic AMP and cyclic GMP levels in ventricular strips exposed to the same physiological conditions. Epinephrine augmented twitch tension, depressed contracture tension, and augmented cyclic AMP levels. Phosphodiesterase inhibitors potentiated the effects of epinephrine. Exogenous cyclic AMP usually mimicked the epinephrine effect on both twitch and contracture tension. Carbachol and acetylcholine depressed twitch tension without affecting contracture, and carbachol transiently augmented cyclic GMP while depressing cyclic AMP levels. These muscarinic agonists inhibited the effects of epinephrine in depressing contracture tension and increasing cyclic AMP levels. Cyclic GMP did not mimic the twitch depressant effect of carbachol but did tend to mimic the cholinergic antagonism of epinephrine. Our observations generally support the idea that cyclic AMP mediates the effects of epinephrine but do not clearly relate cyclic GMP to the action of cholinergic agonists.