1. Urine from goats and pigs after intravenous administration of [14C]trimethoprim contained an unidentified fraction of 'other water-soluble metabolites'. This fraction accounts for 56% of the administered dose in goat urine and 65% in pig urine. 2. A sulphate metabolite comprising the major part of the latter fraction in both goat and pig urine was isolated by gel filtration and paper chromatography and identified by spectroscopic methods as 2,4-diamino-5-(3,4-dimethoxy-5-sulphooxybenzyl)pyrimidine. 3. No hydrolysis of the sulphate metabolite occurred on treatment with aryl sulphatase from (a) Helix pomatia, (b) limpets and (c) Aerobacter aerogenes, while treatment with aryl sulphatase from abalone entrails led to very slow hydrolysis.