The phases of the oestrous cycle in the adult white rat

Since the classical description of Long & Evans (1922) of the cyclical changes which take place in the vaginal smear of the rat, vaginal smea » histology has always been used as an index of ovarian activity. It is also used as a test of the potency of various steroid hormones (e.g. Emmens, 1939), and as a means of correlating cyclical ovarian changes with those in the accessory reproductive organs and other endocrine organs (e.g. Astwood, 1939; Bourne & Zuckerman, 1940). Numerous studies have been made of the cytology of shed vaginal cells by means of trichrome stains (e.g. Hartman, 1944), and various modifications have been suggested for the original technique proposed for taking vaginal smears. Under normal laboratory conditions it is inconvenient to take smears more than once a day. At the same time it is known that frequent smearing may itself induce cornification (Astwood, 1939). In view of the need to time various operative procedures accurately in relation to the phases of the oestrous cycle, the following study was undertaken (a) to determine the individual variability of cycle length in our own colony of rats and (b) to compare our data with those reported for other laboratories