Large superovulatory doses of gonadotrophins result in reduced fertility in laboratory and large domestic animals and it has been postulated that some of the superovulated oocytes are derived from abnormal follicles which would not ovulate under normal physiological stimuli. Follicular growth, follicular maturation and atresia, ovulation and the nidation of the fertilized oocyte require intense tissue remodelation which can be accomplished only through the action of hydrolytic enzymes. We have studied the activities and sub-cellular distribution of three lysosomal enzymes (acid phosphatase, N-acetyl-beta-D-glucosaminidase and beta-glucuronidase) in the follicular fluid, granulosa and theca cells of preovulatory follicles and in the endometrial tissue of immature Wistar rats injected with 4 (control) or 40 (superovulated) IU of pregnant mare serum gonadotrophin (PMSG). Enzyme activities were from four to ten times higher in theca than in granulosa cells. This difference was particularly important in the case of beta-glucuronidase. Large preovulatory follicles tended to have higher activities of lysosomal enzymes in the free fraction of all the compartments studied. This difference was remarkable in theca cells where free enzymes would be required to help ovulation. Forty IU of PMSG induced higher activities of acid phosphatase in theca and granulosa cells than 4 IU, but in endometrial tissue this latter dose of PMSG was more efficient to induce higher activities of this enzyme. The endometrial bound fraction of N-acetyl-beta-D-glucosaminidase was almost three times higher than the free activity. This behavior was also observed in endometrial beta-glucuronidase but only in the control rats. The results observed in follicular fluid were less homogeneous. The activities of glucosaminidase and acid phosphatase were two to three times higher in rats overstimulated with 40 IU of PMSG than in the control rats, whereas the activities of beta-glucuronidase were lower in the superovulated rats. Our results suggest that alterations in the process of tissue remodeling required for ovulation of mature, normal oocytes and for nidation of the fertilized ovum may be important factors to explain pregnancy failure in the PMSG superovulated female.