Cimetidine pharmacokinetics were studied in 13 otherwise healthy but obese volunteers, having a mean body weight of 113 kg and a mean percentage ideal body weight (IBW) of 179%. Sixteen healthy volunteers of normal body habitus (64 kg, 99% IBW) served as controls. All subjects had normal renal function and no laboratory or clinical evidence of hepatic or cardiac dysfunction. After administration of 200-300 mg of cimetidine by rapid intravenous injection, multiple plasma samples obtained over the next 24 h were analyzed for cimetidine concentration by high pressure liquid chromatography. Elimination half-life was not different between obese and control subjects (2.23 versus 2.08 h). Apparent volume of distribution was also similar between subject groups (120 versus 106), as was total metabolic clearance (616 versus 579 ml/min). Using percentage IBW as a measure of obesity, no relationship was found between percentage IBW and apparent volume of distribution (r = 0.29). Cimetidine similarly distributes into IBW in both obese and normal weight subjects, and there is minimal distribution of cimetidine into excess body weight over IBW. Furthermore, there is no difference in total metabolic clearance or half-life of cimetidine between obese and control subjects. Cimetidine dosage in clinical practice should therefore be calculated on the basis of IBW, which better reflects lean body mass, instead of total body weight, which reflects adipose tissue weight in addition to lean body mass.