Right and left ventricular ejection fractions (RVEF and LVEF) were studied at rest, during exercise, and in the immediate post-exercise period (recovery) in 23 patients treated with doxorubicin. Ejection fractions were determined using equilibrium gated radionuclide angiography and non-interactive computer processing. Two effects of doxorubicin emerged: (1) The LVEF at rest and the maximal LVEF reached during the stress test decreased as a function of the cumulative dose expressed in mg/m2. (2) When two measurements were performed before and after an interval dose of less than 100 mg/m2, the resting and maximal LVEF showed an increase, but at higher interval doses, those values showed a decrease. In general, the effect of an incremental dose was not a function of the cumulative dose. No significant changes were detected in RVEF, and LVEF at standardized exercise levels did not correlate with cumulative doses or change as a function of interval dosage.