Beta-adrenoceptor blocking drugs are widely used as effective antihypertensive and antianginal agents. We have determined the effect of beta-blockade in the rat to ascertain whether there are differences between metoprolol (MET) and acebutolol (AC) with respect to regional blood flow (RBF). Both AC and MET were administered as a single or multiple intravenous (iv) doses in Sprague-Dawley rats. Microspheres labelled with (85)Sr and (141)Ce were used to measure cardiac output (CO) and RBF before and after drug administration. CO and RBF were measured 1 and 10 min after the i.v. administration of AC (30 mg/kg) and MET (10 mg/kg). After acute administration of MET, CO decreased by 65% and 31% after 1 and 10 min measurements, respectively. These values were 54% and 28% for AC as compared with baseline values. After chronic administration of either AC or MET, however, there were no significant reductions in CO as compared with saline. Both MET and AC significantly reduced RBF in most organs either after 1 or 10 min measurements when compared with the baseline values. It is concluded that both AC and MET reduced CO and RBF after acute administration. The CO and RBF however, returned to normal after chronic administration.