SummaryThe influence of 1,2 unsaturation, 6a-methylation, 9a-fluorination, 16a-hydroxylation, and 16a,17a-ketalization alone and in combination on glycogenic and thymolytic activity has been simultaneously assessed in immature adrenalectomized rats. Good agreement between glycogenic and thymolytic relative potencies was observed. Unsaturation at positions 1,2 or 6a-methylation, or 16a,17a-ketalization increased glycogenic and thymolytic activity 2- to 3-fold. Presence of a 9a-fluoro group enhanced activity 3- to 4-fold. 16a-Hydroxylation reduced activity by 1/3 to 1/4.The authors are indebted to W. Pierce, E. Ryerson, W. Cook and R. Steinhoff for technical assistance; to J. D. Haynes for statistical analyses; and to the Organic and Biochemical Research Sections for generous supplies of steroids.