The hemodynamic effects of dopamine, (+/-)-dobutamine (racemic mixture) and the (+)- and (-)-enantiomers of dobutamine were evaluated in anesthetized normotensive rats. Dopamine and (+/-)-dobutamine produced hemodynamic effects in anesthetized rat that were qualitatively similar to those reported for these compounds in man. The increase in cardiac output produced by dopamine and (+/-)-dobutamine was due mainly to an increase in stroke volume, with heart rate being only minimally affected. Dopamine produced a large increase in mean arterial pressure and slightly increased total peripheral vascular resistance, whereas (+/-)-dobutamine only modestly increased blood pressure and significantly reduced total peripheral resistance. The (-)-enantiomer of dobutamine, which possesses mainly alpha 1-adrenoceptor agonist activity, produced marked increases in cardiac output, stroke volume, total peripheral resistance and mean arterial pressure, but did not significantly increase heart rate. In contrast, (+)-dobutamine, which possesses predominantly beta 1-and beta 2-adrenoceptor agonist activity, elicited only a modest increase in cardiac output which was due entirely to increased heart rate since stroke volume was not increased. Total peripheral vascular resistance and mean arterial blood pressure were both reduced by (+)-dobutamine, characteristic of a beta-adrenoceptor agonist. The increase in cardiac output and blood pressure produced by (+/-)-dobutamine, but not the positive chronotropic effect, were significantly inhibited by alpha 1-adrenoceptor blockade with prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)