The effects of fentanyl and droperidol on left ventricular performance were evaluated in the neurally intact dog right-heart-bypass preparation under conditions of constant cardiac output, arterial pressure, and heart rate. Fentanyl, .01 and .02 mg/kg body weight, and droperidol, 0.5 mg/kg, did not affect left ventricular performance. However 1.0 mg/kg droperidol caused a significant (P less than .05) increase in left ventricular end-diastolic pressure and a small decrease in maximum left ventricular dP/dt (.05 less than P less than .10). No significant change in myocardial oxygen consumption was observed. This study indicates that large doses of droperidol may depress left ventricular performance and may account for a portion of the hypotension observed after its administration in man. (Key words: Anesthetics, intravenous, fentanyl; Anesthetics; intravenous, droperidol; Heart, function, fentanyl; Heart, function, droperidol.).