The effects of moderate to severe renal impairment on kinetics of the H2-blocker ranitidine were investigated in 16 patients divided into two groups. Mean inulin clearance (ClIn) was 35 +/- 18.3 ml/min/1.73 m2 in group I and 7.4 +/- 3.5 ml/min/1.73 m2 in group II. Each patient received a single 150-mg oral dose of ranitidine. Values determined were maximum plasma concentration (MC) and time of occurrence, AUC, elimination t 1/2 (t 1/2 beta), total amount of unchanged ranitidine recovered in urine, and ranitidine renal clearance (ClR). MC values were higher and longer delayed than values reported in subjects with normal renal function. The t 1/2 beta was longer in group I and group II and correlated with the degree of renal impairment. The amount of ranitidine excreted within the first 24 hr decreased (18% of the dose in group I and 6% of the dose in group II), and ClR correlated strongly with ClIn, indicating that the observed changes in ranitidine kinetics are mainly related to changes in its renal excretion.