Alpha-1-acid glycoprotein (AAG), 750 mg/kg, was administered to rats to determine its effect on propranolol binding and beta blockade. Anesthetized rats received [3H]propranolol i.v., followed in 15 min by human AAG or bovine serum albumin, 750 mg/kg. AAG treatment produced a human AAG concentration in serum of 7.76 ± 1.17 mg/ml, several times higher than the endogenous serum AAG concentration in stressed rats. AAG treatment significantly increased the heart rate response to isoproterenol, compared to albumin (95.4 ± 19.6 vs 28.3 ± 16.7% of baseline value, measured 45 min after propranolol, P < 0.001). AAG-treated rats had greater [3H]propranolol binding in serum (93.0 ± 3.2 vs 76.7 ± 3.0%, P < 0.01) and a lower calculated unbound [3H]propranolol concentration in serum (2.7 ± 1.3 vs 7.4 ± 3.1 × 106 dpm/ml, P < 0.001) than albumin-treated rats. These data demonstrate that AAG can alter propranolol pharmacokinetics and pharmacodynamies even when administered after the propranolol effect is evident. Because the reported affinity of propranolol for cardiac beta receptors is 10,000 times greater than its affinity for AAG, these data suggest that AAG acted by altering propranolol disposition rather than by directly competing with beta receptors for drug.