The vole, Microtus agrestis, was chosen for this study of mast cells during early pregnancy because this species does not show spontaneous estrous cycles. Mast cell numbers in the uterus are known to vary during the estrous cycle in some species (rat, cow, Syrian hamster). Mast cell changes during early pregnancy in the vole could not reflect hormonal changes which had occurred during a preceding estrous cycle. Mast cells in the uterus (myometrium, endometrium, and mesometrial triangle) and ear skin were examined at 0 hours (virgin, estrus) and at 24, 48, 72 and 96 hours postcoitum (p.c.). The stain used was 0.06% toluidine blue in 0.12 M Michaelis's veronal acetate-hydrochloric acid buffer at pH 4.5. The number of mast cells observed in the uterus was not significantly affected when the nondehydrating fixative used routinely ( Helly 's solution) was substituted by a dehydrating fixative (Carnoy's solution without chloroform). The number of mast cells in the myometrium decreased from 0 to 72 hours p.c. and increased from 72 to 96 hours p.c. There was no significant variation in mast cell numbers in the endometrium. The number of mast cells in ear skin and in the mesometrial triangle decreased from 0 to 48 hours p.c. An increase occurred from 48 to 96 hours p.c. in ear skin and from 72 to 96 hours p.c. in the mesometrium.