Cardiac output (CO) and blood flow to major organs were investigated in pentobarbital-anesthetized rats using 85Sr- and 141Ce-labeled microspheres (MS) (15 +/- 1 microns (SD)) injected into the left ventricle. Since MS dissolved in dextran cannot be used in albino rats, the method was evaluated using MS dissolved in Ficoll-70. Two subsequent MS injections were administered to each rat at normal gas atmospheric pressure (Group 1). The results demonstrated homogeneous mixing of MS dissolved in Ficoll-70 and good reproducibility of hemodynamic measurements using ventricular injection. In three other series, control measurements were performed in each rat at a normal gas atmosphere and at atmospheric pressure, and in one of the following experimental situations: at 5.0 atmospheres absolute (ATA) of ambient pressure and normal O2 tension (PO2 - 0.2 ATA) (Group 2); at an ambient pressure of 5 ATA and high O2 tension (PO2 = 1.0 ATA) (Group 3); and at normal ambient pressure and high O2 tension (PO2 = 1.0 ATA) (Group 4). The inert gas was nitrogen. Cardiac output, heart rate, and mean arterial pressure did not change in any group. Renal blood flow fell by 15% (P less than 0.05) in all groups except in Group 1. Myocardial and cerebral blood flow increased by 30% (P less than 0.05) in groups 2 and 3. Blood flow in the diaphragm fell by 40% (P less than 0.001) in Group 4.