The capacity to evoke progestational changes in the uterine endometrium was originally thought to be specific to progesterone, the hormone of the corpus luteum, a substance which is difficult to obtain in quantity, and which is inactive by mouth. Klein and Parkes [1937], however, found that testosterone and androstenedione have a slight progesterone-like activity, while the methyl and ethyl derivatives of certain of the androgens, notably methyl testosterone, have an activity about [unk]th that of progesterone itself. Since methyl testosterone differs from progesterone only in having the groups —OH and —CH3 in position 17 instead of—H and —COCH3 this result, though disproving the supposed complete specificity of progesterone, emphasized the importance of the short side chain in conferring specific activity on progesterone. Recently, an important advance has been made by the addition of an ethinyl group to testosterone, giving a compound which differs from progesterone in the absence