The influence of a pure afterload reduction on systolic time intervals and various echocardiographic indices was assessed in six healthy volunteers, who underwent a single blind placebo controlled trial of three regimens of intravenous dihydralazine (6.25, 12.5, and 25.0 mg). Subsequently autonomic blockade (propranolol and atropine) was used to eliminate the influence of the autonomic reflex action during dosage with dihydralazine 6.25 mg. Dihydralazine increased heart rate to an extent depending on dose, decreased diastolic blood pressure, end systolic diameter, and end systolic wall stress, and enhanced fractional shortening and cardiac output. Heart rate corrected pre-ejection period was shortened and left ventricular ejection time was prolonged, whereas electromechanical systole remained virtually unchanged. Autonomic blockade eliminated dihydralazine's effect on heart rate and cardiac output; blood pressure and systolic wall stress fell appreciably, whereas dihydralazine-induced changes in the systolic time intervals were virtually unaffected. Thus the present study indicates that regardless of the autonomic reflexes heart rate corrected pre-ejection period and left ventricular ejection time, but not electromechanical systole, are highly dependent on afterload, and this effect should be considered when systolic time intervals are evaluated.