The pharmacokinetics and elimination of ciprofloxacin and its three major metabolites desethylene ciprofloxacin (M1), sulfonylciprofloxacin (M2), and oxociprofloxacin (M3) were determined in 18 volunteers with normal and varying degrees of reduced renal functions. One dose of 500 mg ciprofloxacin was given orally. Samples were assayed by high-pressure liquid chromatography. Serum concentrations of both ciprofloxacin and its metabolites were only slightly influenced by the renal function. The serum concentrations of the metabolites were less than 10% of the ciprofloxacin levels, even in reduced renal function, and were overlapping within groups of patients arranged according to renal function. Dialysis reduced the serum concentration of the parent compound and its metabolites. The serum half-life of ciprofloxacin in normal renal function was 5.8 +/- 1.2 h; this rose to 10.8 +/- 2.3 h in the group with clearances of 10-30 ml/min. Compared to the latter group, the t1/2 was lower (7.0 +/- 2.9 h) in the patients with terminal renal failure. The period of monitoring has a distinct consequence for the t1/2 of ciprofloxacin. The shorter t1/2 values emanate if monitoring had stopped after 10 or 12 h. A slower gamma-phase of elimination was observed and this was particularly distinct in subjects with renal functions within the normal range. The total renal elimination of the parent compound and its metabolites was approximately 60% over the 48-hour collection period in normal renal function and was reduced by about 20% in the group with clearances within the range of 10-30 ml/min. In renal impairment, there was a shift towards a higher proportion of the dose being eliminated as M2. M1 contributed only up to 2% of the dose in urine. Irrespective of the renal capacity, the amount of M3 recovered in urine was 3-4%.