1. Diflunisal (DF) is metabolized in rats and humans primarily to its acyl glucuronide, phenolic glucuronide and sulphate conjugates. 2. DF sulphate was synthesized and administered i.v. to conscious bile-exteriorized rats at 10 mg DF equiv./kg. 3. DF sulphate was excreted almost exclusively in urine, and its systemic hydrolysis, monitored by glucuronidation of liberated DF, was quantitatively negligible. 4. Systemic stability of DF sulphate was therefore similar to DF phenolic glucuronide, and contrasted with the systemic instability of DF acyl glucuronide.