The high unitage of gonadotropin in the blood and especially the placenta of the mare during the second, third and fourth months of pregnancy when the fetus is growing from about 2 to 20 cm. (crown-rump length) would seem to be responsible for the secondary, multiple luteal bodies of the maternal ovaries, and the remarkable overgrowth of the fetal gonads. The primary corpus of pregnancy regresses at about the end of the first month, and from then until about the end of the fourth month multiple corpora are found in both ovaries. Cole and others found rupture points in some of the secondary corpora, leading them to the belief that these bodies followed a true ovulation as normally, but histologic studies have led us to believe that the secondary corpora may also be formed by hyperplasia of the theca interna with accompanying degeneration of the ovum and granulosa (Fig. 1). Such thecaluteal bodies may be produced in rodents by injections of pregnant mare's gonadotropin, and appear similar to the aberrant corpora of rhesus monkeys. The secondary corpora usually regress by mid-pregnancy, and the maternal ovaries become atrophic and fibrotic. It was of interest, therefore, not only to determine the progestin-content of the corpora lutea, but also to investigate other possible sites of production or storage of that hormone, especially the placenta in the latter half of pregnancy, since some workers have found progestin in the human placenta. Unfortunately the work on the detection of progestational hornione in the atlreiial cortex had not come to our attention until this study 11 as completed.