Exposure to methoxychlor (MXC) during early pregnancy impairs implantation and reduces serum levels of progesterone. The study was designed to examine the effect of MXC on ovarian steroidogenesis during early pregnancy. Pregnant rats were treated with MXC at 0, 25, 50, 100, 250, or 500 mg/kg/day during Days 1-8 of pregnancy and killed on Day 9. At this time ovaries were removed and incubated in vitro to assess secretion rate of progesterone, estradiol, and testosterone. Steroid hormones in media and serum were assayed 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, but incubating ovaries in vitro revealed a reduction in the rate of ovarian estradiol secretion. The differential changes in serum levels and in vitro whole ovary secretion of progesterone and estradiol may reflect opposing effects of MXC on hepatic steroid hormone metabolism. The authors suggest that MXC appears to accelerate hepatic metabolism of progesterone and reduce the metabolism and clearance rate of estradiol.