Antifertility properties of the pesticide methoxychlor (MXC) are well documented. Administration of MXC to rats during early pregnancy impairs implantation and reduces serum progesterone. The current study was designed to examine the effect of MXC on ovarian steroidogenesis and to define the mechanism(s) by which the pesticide exerts this effect. Rats were treated with MXC at a range of doses during days 1 to 8 of pregnancy and killed on day 9. Ovaries were incubated to assess the secretion of progesterone, estradiol, and testosterone in vitro. Steroid hormones in medium and serum were measured by radioimmunoassay. Although in vivo treatment with MXC reduced serum progesterone, no effect on the ovarian secretion of progesterone was detected in vitro. Conversely, MXC had no effect on serum estradiol levels (testosterone levels were undetectable in serum), but the incubation of ovaries in vitro revealed a reduction in the rates of ovarian estradiol and testosterone secretion.