1 Thirty-three patients with no evidence of endocrine disease, hepatic or renal insufficiency or sleep disorders, were classified in groups 1 to 4 in order of increasing of percentage of ideal body weight (IBW) respectively: less than 90% of IBW, 90--120%, 120--180%, and greater than 180% of IBW. After oral administration of 200 mg butobarbitone, concentration of the intact drug was measured by gas liquid chromatographic assay in urine samples collected during 72 h and at three times in blood. 2 A highly significant negative relationship was found between the cumulative excretion of butobarbitone with urine and the logarithm of the percentage of IBW. In contrast for a given weight, excretion of the drug with urine was found to be weakly correlated with the diuresis. 3 The cumulative urinary elimination of butobarbitone was significantly different between the groups studied, except of the difference between the group 2 and 3 of the patients. No significant difference was found between the renal clearances of butobarbitone in the four groups of subjects. 4 We conclude that redistribution of butobarbitone into adipose tissues can explain the obtained results and that obesity modifies the pharmacokinetics of the drug.