Estrogen has been reported to enhance CA1 functional plasticity in adult rats, as measured by the induction of long-term potentiation (LTP). In the present study, the effects of androgens on CA1 LTP were assessed in adult male Sprague-Dawley rats in vivo following peripubertal castration. Castrated rats with cholesterol implants showed significantly greater plasticity, both in degree and duration of potentiation, than castrated rats given testosterone or dihydrotestosterone implants. An LTP paradigm reported to produce decremental LTP in vitro produced nondecremental LTP in androgen-deprived rats, but decremental LTP in androgen-treated rats. These results suggest that androgens, unlike estrogens, act to reduce CA1 plasticity in the adult rat.