Studies on the physiology of progesterone in the rat have met with difficulties because of the lack of clear-cut anatomic end-points of progestational effects. Detailed studies have heretofore been centred on the rabbit, wherein the transformation of the endometrium and the quiescence of the uterine musculature present striking evidence of corpus luteum hormone action. In the rat, uterine changes during pseudopregnancy, lactation, and after progesterone injections are by no means as striking. The decidual reaction, on the other hand, is a definite and specific indicator of corpus luteum activity in the rat and has therefore yielded the most exact information regarding the endocrine relations of early pregnancy. Because of its high degree of specificity, this response has been adapted to the assay of progesterone. Rats are more readily available in many laboratories than rabbits, and a satisfactory assay method based upon the rat may be useful in certain instances in