The physiological effects of intravenous ouabain on left ventricular (LV) systolic function and metabolic-to-mechanical energy transfer were examined in eight conscious dogs. LV pressure and volume were measured using micromanometers and ultrasonic dimension transducers during transient vena caval occlusions under control conditions and after increasing doses of ouabain. Doppler coronary flow and coronary sinus O2 saturations were used to determine arterial-to-coronary sinus O2 content difference and thereby to calculate LV O2 consumption; total mechanical energy was computed as the sum of LV stroke work and the product of end-diastolic volume and LV mean ejection pressure, neglecting LV unstressed cavitary volume. The slope (10(4) erg/ml) of the stroke work vs. end-diastolic volume relationship increased progressively with rising doses of ouabain from 7.0 +/- 1.6 at control to 9.6 +/- 1.7 after ouabain 0.75 mg (P = 0.0002). Regression analysis of LV O2 consumption (mW/cm3) vs. total mechanical energy (mW/cm3) yielded a linear relationship that did not change with 0.75 mg of ouabain (P > 0.4). These data indicate that ouabain possesses a significant positive inotropic effect on the intact left ventricle without a change in energy transfer efficiency or O2 wasting.