This study was designed to compare the effects of equivalent therapeutic doses of two H2 antagonists, cimetidine and ranitidine, on theophylline pharmacokinetics and to determine whether the previously described cimetidine-theophylline interaction is dose dependent. Twelve healthy adult men were given a 6-mg/kg intravenous aminophylline dose on four occasions. Subjects were randomly assigned four treatments: no treatment (control); cimetidine, 1,200 mg/day; cimetidine, 2,400 mg/day; and ranitidine, 300 mg/day. Cimetidine, 1,200 mg/day, significantly decreased theophylline clearance by 36% (range, 22% to 49%) and increased the mean elimination half-life from 5.7 hours (control) to 9.2 hours. A significant difference was not found between the two cimetidine dosages, indicating dose independence of the interaction over the dosage range studied. Ranitidine did not significantly alter theophylline pharmacokinetics. Theophylline plasma protein binding was not affected by any treatment. The relative effects of cimetidine and ranitidine on the elimination of cytochrome P-450 metabolized drugs such as theophylline indicate a useful property of ranitidine as compared with cimetidine.