The pharmacokinetics of ciprofloxacin has been studied after a single oral dose of 500 mg given to 5 normal subjects (N) and to 15 patients grouped according to their residual renal creatinine clearance: Group I, 8-30 ml X min-1, Group II, less than 8 ml X min-1, and Group III, haemodialysed patients studied twice--during an interdialysis period (IIIa) and in a 4 h haemodialysis session (IIIb). Ciprofloxacin was assayed by reverse phase HPLC using a spectrofluorimetric detection. The peak plasma concentration (2-5 mg X l-1) was reached within 2 h after drug administration. Apparent volume of distribution, 6.6 (N), 5.0 (I), 2.7 (II) and 4.2 (IIIa) l X kg-1 and total plasma clearance, 770 (N), 440 (I), 378 (II) and 314 (IIIa) ml X min-1 were decreased in relation to the degree of renal impairment. Mean plasma half-lives for patients in the 4 groups were 7.3 (N), 10.4 (I), 7.2 (II) and 9.3 (IIIa) h. In groups N, I and II, 40, 16 and 8% of the administered dose was eliminated through the kidney, with mean renal clearances of 305 +/- 63, 61 +/- 21 and 21 +/- 3 ml X min-1. A linear relationship was found between the renal clearance of ciprofloxacin and the glomerular filtration rate (r = 0.75, n = 15). Ciprofloxacin was partly removed by haemodialysis (IIIb): the dialyser extraction ratio was 23% and the dialysis clearance was 40 ml X min-1.