The effects of phenoxybenzamine HCl and propranolol HCl, 2 mg/kg, on tissue oxygen tension (PO2), perfusion and small vessel blood content of the cerebral cortex and biceps brachii muscle of anesthetized rats were determined. Perfusion and PO2 were measured polarographically and small vessel blood content was measured with 59Fe-siderophilin-labeled blood. Under control conditions PO2, perfusion and small vessel blood content averaged 15.1 mm Hg., 15.6 ml/min/100 g and 0.91 ml/100 g in brain and 15.6 mm Hg, 13.1 ml/min/100 g and 1.63 ml/100 g in muscle. After phenoxybenzamine adminstration, there was a significant increase in muscle perfusion (17.4%) and decrease in cortical PO2 (9.2%). No other factors changed significantly. Propranolol caused no significant changes in any of the above factors. Arteriolar resistance in skeletal muscle decreased after phenoxybenzamine. Small vessel blood content measurements (an estimate of open capillary density) indicate no effects on precapillary sphincters with either agent. Since some changes in metabolism were indicated with these agents, regional oxygen consumption was calculated from this data.