1. The metabolism of sodium cortisone 21-[35S]sulphate was investigated in rats. 2. Quantitative and qualitative experiments showed that substantial amounts of 35SO42- appeared in the urine of free-ranging rats receiving the ester. 3. Whole-body radioautograms indicated considerable biliary elimination of 35S and also pointed to the liver as the site of metabolism. 4. When female rats with bile-duct cannulae received sodium cortisone 21-[35S]sulphate approx. 70% of the dose appeared in the bile as a doubly conjugated steroid (metabolite I). This metabolite was identified as 3α-(β-d-glucopyranuronosido)- 17α-hydroxy-21-[35S]sulpho-oxy-5α-pregnane-11,20-dione. 5. When metabolite I was administered to a rat with a bile-duct cannula 90% of the dose appeared in the bile unchanged. After the administration (intraperitoneally or orally) of metabolite I to free-ranging rats considerable amounts of 35SO42- appeared in the urine. 6. The route by which 35SO42- might be produced from cortisone [35S]sulphate in free-ranging animals is discussed.