SUMMARY The steroids excreted by patients receiving oral prednisone and oral prednisolone have been investigated. The two compounds present in largest amount were isolated and identified as prednisone and prednisolone. Present in smaller amounts were the corresponding 20β-alcohols. A fifth compound had properties suggestive of pregna-1:4-dien-17α:20α:21-triol-3:11-dione. There is no evidence that prednisone and prednisolone are converted in the body to cortisol, cortisone, or their derivatives, and the investigations suggest that prednisone is metabolized in part by reduction of its 11-ketone group to a 11-hydroxy group and prednisolone by oxidation of its 11-OH group to an 11-ketone group. These two compounds are thus metabolically inter-convertible. Reduction of the 20-ketone group can occur in both compounds.