Direct measurements of arterial pressure, stroke volume (SV), and cardiac output (CO) were obtained in ether-anesthetized rats with established spontaneous hypertension (SHR) treated with alpha-methyldopa and compared to both untreated hypertensive and normotensive Wistar-Kyoto (WKY) rats. Left ventricular pumping ability was determined by the maximum levels of SV and CO reached during rapid intravenous volume loading with blood. Treatment with methyldopa reduced the SHR arterial blood pressure (average 57 mm Hg) and reversed the cardiac hypertrophy toward normal. In comparison to untreated SHR, therapy increased heart rate and CO and decreased peripheral resistance. During volume-loading, the levels of SV and CO at matched left ventricular end-diastolic pressures were significantly higher in treated vs untreated SHR. To evaluate the role of blood pressure in the improved peak pumping ability observed in treated rats, a phenylephrine infusion was used to equalize pressures while repeating cardiac function studies. In normotensive WKY and untreated SHR, left ventricular pump function was not greatly affected. A pronounced depression in peak SV and peak CO was observed only in treated SHR. The data indicate that treatment with methyldopa is associated with improved ventricular function in part related to the reduction in arterial pressure.