The pharmacokinetics of orally administered quinine were determined in six normal volunteers before and after a 7-day course of cimetidine (1 g day-1) or ranitidine (300 mg day-1). Peak plasma quinine concentration and the time of peak concentration were not altered after cimetidine or ranitidine pretreatment. After cimetidine pretreatment there was a significant reduction in the apparent oral clearance of quinine, from 0.182 +/- 0.063 (mean +/- s.d.) to 0.133 +/- 0.055 1 h-1 kg-1 (P less than 0.05). This was reflected in a 49% (range 17 to 90%) increase in the mean elimination half-life from 7.6 +/- 1.3 to 11.3 +/- 3.7 h (P less than 0.05). In contrast to cimetidine, ranitidine had no significant effect on the clearance or half-life of quinine. The apparent interaction between quinine and cimetidine may have therapeutic implications. Special care should be taken in patients taking these two common drugs concomitantly. Additionally, to avoid unnecessary risks due to drug interaction, the use of ranitidine may be preferable in the patients in whom it is desirable to administer an H2-receptor antagonist together with quinine.