Previous studies have reported increased total myocardial blood flow (TMBF) after 15 min stable pressure in a normoxic O2-N2 and O2-He at 5 bar, although cardiac output (CO), heart rate (HR), and mean arterial pressure (MAP) remained unchanged. In the present study, 2 groups of awake rats were exposed to normoxic 5 bar atmospheres; group 1 breathing a He-O2-N2 mixture and group 2 a O2-N2 mixture. Organ blood flow was determined by the microsphere method in control (C) (group 1) and after 15 (T1) and 75 min (T2) hyperbaric exposures (groups 1 and 2). MAP and HR remained at control levels in group 2 animals and increased slightly (6-10%, P less than 0.05) in group 1 rats. CO remained unchanged during the experimental period in both groups. In group 1, TMBF had increased by 13% (P less than 0.05) at T1 and continued to increase at T2 for both groups. Blood flow to the liver and spleen increased during the exposure in both groups. Renal blood flow fell by 25% from C to T2 (P less than 0.05). The arterial blood gases and pH remained at the predive control level in group 2 rats, whereas serum corticosterone concentration fell to 60% during compression (P less than 0.01), possibly due to N2 narcosis, but increased gradually toward 80% of C value during the pressure exposure. We conclude that the increase in TMBF, which is initiated by compression to a 5-bar normoxic atmosphere, persists when stable pressure is maintained.