The change in distribution of cardiac output in septic shock was examined by radionuclide imaging with thallium 201 thallous chloride (201Tl) which allows noninvasive evaluation of relative blood flow to various organs except for the brain. Pseudomonas aeruginosa (1 X 10(9)-2 X 10(10) organisms) were inoculated into the thigh of rats 18-24 hr before the study. The mean arterial pressure was measured with an intracarotid catheter. Fractional blood flow to the heart, kidneys, and liver was evaluated as organ uptake of 201Tl. Those with zero or less than 5% kidney uptake (n = 8) had a high heart uptake and all died within 3 hr even if their pressure was maintained. In contrast, 20 out of 24 rats with kidney uptake greater than 5% survived for more than 6 hr. Those results suggest that the kidney uptake, representing fractional renal blood flow, is an excellent indicator of short-term prognosis in septic shock.