To investigate dobutamine-induced cardiac adaptations, we compared dobutamine-treated with swim-trained and sedentary control rats. After 14 wk of treatment, heart rate was lower in the dobutamine-treated (279 +/- 6) and exercise-trained (287 +/- 4 beats/min) groups than in the control animals (305 +/- 3 beats/min; P less than 0.05). The exercised rats gained less weight (164 +/- 11 g; P less than 0.05) than the dobutamine-treated (238 +/- 16 g) and the control animals (231 +/- 12 g). Also, compared with the two other groups, the exercise group had higher relative heart weights (3.47 +/- 0.08 vs. 2.82 +/- 0.06 and 2.90 +/- 0.05 g/kg in the dobutamine and control groups, respectively; P less than 0.05) and lower epididymal fat pad weights (6.6 +/- 0.4 vs. 13.3 +/- 1.0 and 11.4 +/- 0.5 g/kg in the dobutamine and control groups, respectively; P less than 0.05). However, both the maximum heart rate produced by isoproterenol and the isoproterenol dose producing 50% of the peak heart rate response were similar among the three groups. Myocardial norepinephrine content, beta-adrenergic receptor number, and adenylate cyclase activation by isoproterenol, NaF, 5'-guanylyl imidodiphosphate, and forskolin also did not differ. Thus, although there were differences between the dobutamine-treated and the exercised rats, the two groups were similar in that they developed bradycardia that was not due to cardiac adrenergic desensitization.