The effects of alpha receptor blockade on cardiac output distribution in pigs were studied. Recordings were made during spontaneous breathing (SB), during ventilator treatment with 8 Pa positive end-expiratory pressure (8 PEEP), after alpha blockade during SB (SB-alpha) and at 8 PEEP (8 PEEP-alpha). The microsphere method was used for blood flow determinations. The animals received either 5 ml.kg-1.h-1 (Group A) or 10 ml.kg-1.h-1 (Group B) of fluids. In Group A on SB-alpha, CO was maintained due to tachycardia but mean arterial pressure (MAP) decreased, renal blood flow and urine production decreased. At 8 PEEP-alpha, CO decreased despite increased heart rate (HR), MAP decreased alarmingly, renal blood flow decreased, urine production ceased and cerebral blood flow decreased, reflecting failing autoregulation. In Group B, CO increased during SB-alpha, SVR decreased, myocardial blood flow increased and organ blood flows were otherwise unchanged. At 8 PEEP-alpha, MAP, SVR, renal, pancreatic and splenic blood flows decreased in Group B. Gastric, intestinal and muscular blood flows were unchanged at 8 PEEP-alpha in Group B which is interpreted as an effect of the alpha blockade. In both groups peripheral arterio-venous shunting increased after alpha blockade.