Quantitative changes have been observed in the catalase activity during follicular growth, induced atresia and in corpora lutea of cycle and pregnancy. Large growing and preovulatory follicles showed higher enzyme activity as compared to the smaller follicles; the activity was mainly present in the thecal layer of the preovulatory follicle. After the blockade of ovulation with barbiturate, the activity increases significantly in the whole follicle and also in the thecal layers till third day of ovulation and afterward it declines on 5th day, suggesting that rise in catalase activity may exert a protective function against lytic actions of peroxide which is known to be produced in the ovary during several metabolic and steroidogenic events. The corpora lutea of the cycle showed significantly less enzyme activity than the corpora lutea of pregnancy. The significance of catalase activity during follicular and corpus luteum degeneration is discussed.