The inotropic responsiveness of the developing myocardium to dopamine and isoproterenol was evaluated using isolated, perfused ventricles and atrial strips from puppies ages 15 hr to 33 days. Responses were compared to those in adult animals. The maximum percentage of increase of left ventricular dF/dt increased from 12 +/- 5 (mean +/- SEM) at 0-7 days (n = 6) to 100 +/- 40 at 21-33 days (n = 3) of postnatal age. At 7-14 days (n = 4) and 15-20 days (n = 5) of age the maximum percentage of increase of left ventricular dF/dt was 28 +/- 10 and 39 +/- 17, respectively. Puppy ventricle responded to isoproterenol at all ages equally (maximum percentage increase of left ventricular dF/dt = 46 +/- 13). Atrial muscle strips from puppies, ages 15 hr to 35 days (n = 22), and adult dogs (n = 19) demonstrated equal responsiveness to dopamine as well as to isoproterenol. The maximum percentage of increase of dF/dt was 117 +/- 18 with dopamine. Maximum percentage of increase of dF/dt with dopamine after propranolol (10(-7) M) was 52 +/- 18. Haloperidol (1.10(-7)-2.10(-6) M) did not alter responsiveness of atria to dopamine.