The beta-blocking activity of propranolol was studied on the positive inotropic effect of norepinephrine, epinephrine, isoproterenol, dopamine and ethylephrine, in the left atrium driven at different rates. Dose of the antagonist of 10(-8) and 10(-7) M did not block the norepinephrine dose-response curve at 1 and 2.8 Hz, but a shift to the right was observed at 1.6 Hz. Although epinephrine showed a significant increase in its pD2 (p less than 0.001) at the lower stimulation frequency, the blockade increased progressively the higher the rates. On the other hand, propranolol antagonized isoproterenol at all the frequencies tested, in spite of an increase in the maxima at 2.8 Hz. This latter behavior was also true for dopamine and ethylephrine. Both sympathomimetic amines were blocked by propranolol at 1 and 1.6 Hz. The complex effect of propranolol on Ca2+ movements and its effects on cAMP and ATPase seem to be superimposed to the beta-blocking activity. Thus, the various actions on the sympathomimetic amines change according to the agonist considered and the stimulation frequency employed.