MIESCHER et at. [1936, 1, 2] found that the activity of testosterone on male sexual organs was greatly increased by esterification. Of the various esters prepared the propionate, as judged by short-duration experiments, proved to be the most effective. It brought about a greater increase in the size of the male sexual organs in castrated rats and also a longer duration of the effect than was obtained with other esters; e.g. after a single injection of 2 mg. of testosterone propionate, dissolved in 2 ml. of sesame oil, or after two injections of 1 mg. in 1 ml. of sesame oil with a 5-day interval, the maximum effect was obtained on about the 11th day after the first injection, the activity lasting at least 2-4 weeks. With testosterone the activity had disappeared by the 11th day, the maximum effect being observed a few days after the injection. Parkes [1936] also confirmed the results quoted above in short-duration experiments on the prostate and seminal vesicles. Such properties of the propionate would be important from a therapeutic viewpoint and it is also possible that testosterone is normally present in the body as an ester. On the basis of the experiments with the esters of testosterone Miescher et al. [1936, 2] confirmed the conclusion which we have repeatedly emphasized in our

THE "sexual hormones" named in the title, which have been prepared artificially, but have not so far been found in the organism, are included in our series of experiments, since they are significant (particularly androstenedione) as probable intermediate products in the biological formation of the male and female sexual hormones from cholesterol [Ruzicka & Wettstein,