1. Age-dependent changes in urinary excretion of zenarestat and the effect of gonadectomy and androgen treatment were investigated in Sprague-Dawley rats. 2. Urinary excretion of zenarestat scarcely differed between 3-week-old male and female rats; it decreased in males from 4 weeks of age, but in females urinary excretion of the drug was essentially constant during ageing. 3. Castration of male rats at 22 days of age abolished the adult sex difference in urinary excretion of zenarestat, while castration at 5 weeks of age produced urinary excretion of the drug about half that in females. 4. Ovariectomy of females at 22 days or 5 weeks of age had no effect on the urinary excretion of zenarestat. 5. Treatment of male and female gonadectomized rats with testosterone resulted in the urinary excretion of zenarestat characteristic of intact adult male rats.