We investigated alterations in the number and affinity of cardiac beta-adrenergic receptors during hemorrhagic shock. Forty male Wistar rats were divided into two groups: (1) a shock group (n=20), in which mean arterial blood pressure was decreased to 40\2-50 mmHg by bleeding and kept constant for 6 h; and (2) a control group (n=20), which underwent a sham operation. We used (\t-) [3H]dihydroalprenolol for the determination of the number and affinity of beta-adrenergic receptors in myocardial membranes. An additional 2.5 rats were used for determination of plasma epinephrine and norepinephrine concentrations. Scatchard analysis showed a 20% reduction (P<0.05) in beta-adrenergic receptor density in the shock group (70.3\+-3.5 fmol-mg\t-1 protein) compared to the control group (90.0\+-4.8 fmol-mg\t-1 protein) but no significant change in the affinity (2.52\+-0.06 vs. 2.31\+-0.09 nmol.l\t-1, control vs. shock). Plasma catecholamine concentrations were increased significantly at 1, 2, 4 and 6 h after the start of hypotension. These data suggest that increased levels of plasma catecholamines in hemorrhagic shock may be correlated a significant loss of beta-adrenergic receptors in rat myocardium.