1. 5-Hydroxytryptamine-O-glucuronide has been extracted from sheep urine and characterized by its chemical reactions and R(F) values on paper chromatograms.2. 5-Hydroxytryptamine-O-glucuronide in urine was separated from an inhibitor of beta-glucuronidase using an anion exchange resin, and then estimated by measurement of the 5-hydroxytryptamine (5-HT) liberated on incubation with beta-glucuronidase.3. About 20% of the 5-hydroxyindoles found in sheep urine was 5-HT conjugated with glucuronic acid (1.18+/-0.08 mg 5-HT/24 h, thirty experiments).4. Sheep urine also contained free 5-HT (0.35+/-0.04 mg/24 h, thirty experiments) and 5-hydroxyindolylacetic acid (4.43+/-0.33 mg/24 h, thirty experiments).5. 5-Hydroxytryptamine-O-sulphate was not found in sheep urine.