1. Rat shows a marked sex difference in the excretion of 14C-zenarestat: only 1% of the dose was excreted in the urine of males, about 45% of the dose was excreted in the urine of females. 14C in the urine of female rats was almost entirely unchanged drug. 2. Plasma protein binding was similar in both sexes: 99.3-99.5% in males and 99.4-99.6% in females. 3. The type and ratio of metabolites in the faeces and bile were not significantly different between males and females. 4. Renal clearance experiments, and inhibition of urinary excretion by probenecid, indicated that female rats may possess an active secretory mechanism which is lacking or relatively inactive in male rats.