AbstractIsoprenaline and dopamine increased cyclic AMP (cAMP) content and contracile activity of isolated perfused rat hearts. The changes of cAMP levels depended on the mode of drug administration. Isoprenaline (4 × 10‐10mol) administered to the perfused heart as a relatively concentrated bolus, caused a substantial, rapid and transient increase of cAMP. Isoprenaline (2 × 10‐10mol and 4 × 10‐10mol) and dopamine (10‐7mol) diluted in 40 ml of perfusate which was continuously recirculated through the heart, caused a gradual increase of cAMP content which approached an apparent steady state. cAMP accumulation occurred at isoprenaline concentrations above 10‐9M and at dopamine concentrations above 10‐6M. Both agents also increased cAMP labelling from14C‐adenine in the perfusate, probably indicating increased cAMP synthesis. Isoprenaline at 2 × 10‐8M and 10‐7M increased labellingmorethan content of cAMP. Isoprenaline and dopamine also increased phosphorylaseaactivity. An association between increased cAMP contents and increased contractile activity was revealed by both the time‐response and the dose‐response curves of hearts exposed to isoprenaline and dopamine. Since both agents stimulate adrenergic β‐receptors in cardiac muscle, the results are concordant with the hypothesis that cAMP is involved as a mediator of the inotropic response