The pharmacokinetics of ampicillin was studied in 38 patients with acute pyelonephritis in the second and third trimesters of pregnancy after the first and the last (28th-32nd) intramuscular injections of the antibiotic. The ampicillin levels in the blood and urine were determined with the agar diffusion method. The pharmacokinetic parameters were estimated in a one-compartmental model by computer The ampicillin levels in the blood and urine of the patients did not practically differ at all the investigation periods (0.5-6 hours after the antibiotic administration) in the second and third trimesters of pregnancy. During the treatment, the rate of ampicillin elimination from the host increased and the period of half-elimination from the blood decreased. The antibiotic levels in the urine within 4-6 hours after the last injection were practically lower in the second trimester of pregnancy as compared with the second trimester. The therapy resulted in an increase in the antibiotic renal clearance, which returned to normal in the second trimester of pregnancy and remained under normal in the third trimester of pregnancy. The increase was due to an approximately 2-fold acceleration of the rate of ampicillin secretion by the renal tubules. The total clearance of ampicillin practically increased in the second trimester of pregnancy and remained decreased in the third trimester of pregnancy. The estimation performed in accordance with the Krueger-Timmer principles on the basis of the characteristic features of the pharmacokinetics of ampicillin shown in the study provides recommendation of the following scheme for pyelonephritis treatment in pregnant women: 500 mg of ampicillin injected intramuscularly every 6 hours followed by gradual decreasing of the intervals between the injections to 4 hours as the rate of ampicillin elimination increases.