The distribution of cardiac output (CO) was studied in conscious rats during hyperbaric oxygen (HBO) exposure to 5 bar for 60 min. Heart rate (HR) and arterial blood pressure (ABP) were continuously recorded. Organ blood flow and CO were measured during control, after 5 and 60 min HBO by the microsphere method. Arterial acid-base balance was measured both during control and HBO. During HBO the animals remained calm and seemed unstressed with no sign of convulsions. The arterial pH remained at control level, whereas PCO2 and HCO3 fell (P less than 0.05). The systolic ABP increased gradually during HBO, while the mean ABP was unchanged. HR started to fall at 1 bar O2 (P less than 0.05), fell markedly during compression (P less than 0.02), and remained below control level during the HBO. After atropine was given, HR increased to predive control in all animals. CO fell by 25% due to increased total peripheral vascular resistance of 27%. Blood flow to most organs fell correspondingly to CO, indicating an equally distributed vasoconstriction. Unchanged blood flow was found in the kidneys, the adrenals, and the liver. Blood flow to the right and left heart ventricles fell significantly more than the average reduction of CO. Calculation of a pump work index of the heart by the product of systolic ABP and HR showed unchanged performance, while the blood flow to the left ventricle fell by 50%, which suggests insufficient O2 supply.