Carbonic anhydrase activity was localized histochemically in the ovary, fallopian tube, and uterine corpus of seven mammalian species: mouse, rat, guinea pig, rabbit, cat, dog, and human. The enzyme was present in fallopian tube and uterine epithelium and non-vascular smooth muscle, although there was considerable species variation. With the exception of the cat and rabbit, non-erythrocytic activity was seen in the granulosa cells of the ovary and in all species reaction product was present in the surface epithelium. In the cat, no other ovarian sites of activity were present, but, in the rabbit, the enzyme was demonstrated in the oocyte of maturing follicles. Biochemical assays confirmed the occurrence of nonerythrocytic carbonic anhydrase in the guinea pig ovary. The presence of the enzyme in multiple sites throughout the female reproductive tract suggests its importance in reproduction and raises the possibility that inhibitors given during particular stages of conception might be useful as antifertility agents.