The present investigation was undertaken to ascertain the histological appearances of the ovaries of animals killed during lactationi special attention being directed to the luteal tissue.

The material employed consisted of the ovaries of a consecutive series of twenty rats which were killed in the puerperium, at periods varying from thirty-six hours to twenty-three days after parturition. The ovaries show very uniform appearances in the entire series. The corpora lutea are large and visible to the naked eye as reddish excrescences on the surface of the ovary. On microscopic examination they present the size and appearance of the corpora lutea of pregnancy, being much larger than the corpora lutea which develop in the absence of pregnancy or lactation. It is obvious that this must be explained in one of the two ways:

(a) These bodies are the corpora lutea of pregnancy which do not degenerate in the later part of gestation in the manner commonly accepted, but persist throughout the lactation period; or:

(e) They are developed from follicles which have ruptured at or shortly after parturition, and have attained a larger size than obtains in animals which are not nursing.

It is important to note that the corpora lutea of a rat thirty-six hours after parturition do not differ in size or appearance from those of animals killed when their young are weaned at the age of three weeks. This fact inclines me to the view that they are the corpora lutea of pregnancy. Whichever explanation turns out to be the correct one, the state of the ovaries described appears to indicate that the luteal tissue is functionally active during lactation.