Oral ranitidine was given to 68 healthy subjects between 18 and 75 years old at a dosage of 150 mg twice a day for seven doses. Fifteen subjects were 18 to 35 years old (group I), 19 subjects were 36 to 50 years old (group II), 19 subjects were 51 to 65 years old (group III), and 15 subjects were 66 to 75 years old (group IV). Venous blood samples were drawn and the AUC from 0 to 12 hours, the maximum plasma concentration, the time of the maximum plasma concentration, the minimum plasma concentration, and the elimination t1/2 were determined. When groups III and IV were compared with groups I or II, significant (P less than 0.05) increases were seen in the AUC(0-12) (42% and 50%), the maximum plasma concentration (36% and 41%), the minimum plasma concentration (91% and 85%), and the elimination t1/2 (29% and 33%). Positive linear correlations were found when the AUC(0-12) (r = 0.68; P less than 0.01), the maximum plasma concentration (r = 0.34; P less than 0.01), the minimum plasma concentration (r = 0.55; P less than 0.01), and the elimination t1/2 (r = 0.46; P less than 0.01) were regressed with age. Our results suggest that it may be appropriate to consider dosage adjustments for patients over 50 years of age who take ranitidine.