1. To investigate the effect of cimetidine on the pharmacokinetics of R(-)- and S(+)-ibuprofen, six healthy male volunteers received orally 800 mg racemic ibuprofen both in the drug-free state (control phase, C) and on the second day of a 3 day course of oral cimetidine, 1 g daily (treatment phase, T). The two phases (14 days apart) were randomised in a balanced cross-over manner. 2. The plasma concentrations of R(-)- and S(+)-ibuprofen were measured by high-performance liquid chromatography (h.p.l.c.). The protein binding of the enantiomers was assessed in a selection of plasma samples from each volunteer. Following alkaline hydrolysis of glucuronide conjugates, the urinary recoveries of ibuprofen and its major metabolites were measured by h.p.l.c. 3. There was no difference (P greater than 0.05, two-tailed Student's t-test; data expressed as mean +/- s.d.) between C and T phases in the total area under the plasma concentration-time curve of R(-)-ibuprofen (C 4514 +/- 1063 mg 1(-1) min vs T 4665 +/- 1435 mg 1(-1) min) and S(+)-ibuprofen (C 6460 +/- 1063 mg 1(-1) min vs T 6886 +/- 1207 mg 1(-1) min). Similarly, for each enantiomer, there was no difference between the two phases in the terminal half-life, the maximum plasma concentration or the time of its occurrence. 4. Cimetidine treatment had no effect (P greater than 0.05) on the time-averaged percent unbound in plasma of R(-)-ibuprofen (C 0.419 +/- 0.051% vs T 0.435 +/- 0.060%) and S(+)-ibuprofen (C 0.643 +/- 0.093% vs T 0.633 +/- 0.053%). (ABSTRACT TRUNCATED AT 250 WORDS)