The pharmacokinetics of imipenem (MK-787) and cilastatin (MK-791) were studied in 30 patients with cystic fibrosis (CF) receiving the drug for therapeutic purposes at doses of 11 mg/kg given as a 30-min infusion. The serum concentrations and urine elimination were studied after the first dose and during steady state. The concentrations were assayed by high-pressure liquid chromatography. The total areas under the imipenem serum concentration curves (AUCs) to infinity were 30.4 +/- 6.8 mg.h/l after the first dose compared with 29.1 +/- 7.1 mg.h/l during steady state (NS). The cilastatin AUCs on the 2 days were 40.3 +/- 9.3 and 38.3 +/- 0.4 mg.h/l (NS), respectively. The urinary recovery of imipenem was 47.8 +/- 17.8% after the first dose and 57.8 +/- 24.2% during steady state (NS). The amounts of cilastatin eliminated in the urine during 6 h were 6/7.3 +/- 22.9% after the first dose and 60.5 +/- 17.0% of the dose during steady state (NS). The mean half-life of imipenem in these CF patients was 1.2 +/- 0.4 h on the first day of the examination and within the same range during steady state. The distribution volume (Vd beta) was in the range of 28 liters, the total body clearance was 16.3 liters/h (285 ml/min) on the first day. The t1/2 of cilastatin was 0.59 +/- 0.14 h after the first dose and 0.61 +/- 0.14 h during steady state. Thus patients with CF eliminated cilastatin more quickly than imipenem.