The effects of exogenous testosterone enanthate, a long-acting testosterone ester, on estrous activity and reproductive performance in female rats were studied. An injection of 2.5-20.0 mg of testosterone enanthate delayed until 26.940.1 days the cycling of the estrous cycle in a dose-dependent way; the treated rats began regular 4-day estrous cycles 41.W0.7 days after the injection. Subsequent reproductive performance was not different in the treated and control animals (P > .05). Ovarian weight during estrous inhibition decreased steadily after treatment, but the uterine weight increased for a time. The ovaries contained follicles and interstitial cell islands, but no new ovulatory corpora lutea. The injection decreased the number of small follicles (250-549 pm), but increased the number of mature large follicles (2550 pm; P < .05). The mean testosterone concentration in the peripheral plasma during estrous inhibition was higher than that before the treatment (P < .05). Testosterone concentrations increased for a time after treatment ( P < .05), but decreased steadily thereafter. These results suggest that testosterone enanthate might be used for long-term contraception; the effects seem to depend on testosterone. Continuous stimulation by the long-term activity of testosterone inhibited the growth of small follicles but stimulated the growth of large follicles.