The acute effects of three doses ofcaptopril(12.5, 25, and 50 mg [approximately 0.5, 1.O, arid 2.0 mg/kg]) on several haemodynamic variables and plasma aldosterone concentration were investigated in four dogs with experimentally produced heart failure (rapid ventricular pacing) and one dog with dilated cardiomyopattiy. Haemodynamic variables were measured with a Swan‐Ganz ttierniodilution catheter and an indwelling carotid artery catheter at baseline and 1, 2, and 4 h after drug administration. A statistically significant (P<0.0.5) decrease in peripheral vascular resistance was observed 1 and 2 h following the 12.5 nig dose. A significant and large enough decrease in peripheral vascular resistance to produce a significant decrease in mean systemic arterial blood pressure was observed 1 and 2 ti after administering 25 and 50 mg of captopril.A mild but significant increase in cardiac output was observed 1 h after each dose. The drug effect on systemic arterial blood pressure lasted less than 4 h. No statistically significant changes were observed for the group in pulmonary capillary wedge pressure, right atrial blood pressure, or plasma aldosterone concentration at any time. We conclude that the acute haemodynamic benefits provided by captopril atlministration were mild and due primarily to arteriolar dilation. Doses of approximately 1–2 mg/kg produced slightly greater arteriolar dilation than an approximate dose of 0.5 mg/kg. The drug effect was short‐lived, lasting less tha