The radionuclide scintiphoto study, a noninvasive technique, is abnormal in acute pyelonephritis. A delay in excretion of the isotope has also been observed in dehydrated studies in chronic pyelonephritis. To further elucidate this abnormal pattern in pyelonephritis, experimental renal infection was produced in monkeys. Dehydrated 131-I-hippuran scintiphoto studies were conducted weekly for 8 weeks after the initial infection. The scintiphoto studies showed significant delay in excretion of radionuclide in all bacteriuric monkeys at 1 week. After this time, delayed excretion was seen in only those monkeys later classified as having chronic pyelonephritis. This was statistically significant (P smaller than 0.01).