In this study the distribution of mast cells in the cephalic, middle, or caudal portions of the uterine horns did not differ. In cross-sections, however, mast cells were most numerous in the myometrium, intermediate in the tunica vasculare of the endometrium and least in the mucosa. In the endometrium, the mast cells were most numerous in the antimesometrial, least in the mesometrial and intermediate in the lateral quadrants. In the myometrium, however, the relative numbers of mast cells between the quadrants varied according to the day in the estrous cycle.

On days 1 (the day after estrus), 2, 3, and 4, (the day of proestrus) of the estrous cycle, there were respectively 42.7, 21.8, 22.5 and 10.7 mast cells in the endometrium per 12 μ sections. The differences in means between groups was highly significant (P < 0.001). In mated females on 67, 79, 91, 103, 115, and 151 hours after ovulation, there were respectively 9.4, 8.4, 4.7, 5.6, 2.1, and 2.2 mast cells in the endometrium per 12 μ cross-section. The differences in the means were not significant.

The mean number of mast cells per 12 μ cross-sections of uteri of ovariectomized females was 20 ± 8. Subcutaneous injections of 0.3 gamma estrogen did not significantly alter the number of mast cells but 0.6 gamma depressed the mean count to 11 ± 7.

It appeared from these studies that the uterine mast cell numbers were related to the cyclic phenomena of estrous, but that the numerical relationship to the time of nidation was not clear.