Although the fetuses of rats given alcohol prior to and during gestation are small, the placentas are large compared to those of untreated rats. It has been suggested that placental enlargement may be a consequence of a reduction in progesterone production. To investigate this, rats were given 20% ethanol in water prior to pregnancy and 30% ethanol in water throughout gestation, with rat chow ad libitum (alcohol group) or water with an equicaloric diet in which corn starch was substituted for alcohol (pair-fed group), or rat chow and water ad libitum (as libitum control group). Serum progesterone and testosterone were measured on day 17 of gestation and plasma and pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) on day 21 of gestation. Progesterone was significantly reduced in the alcohol but not the pair-fed group. Testosterone levels were not different among the three groups. Plasma LH was significantly reduced in both the alcohol and the pair-fed groups, but plasma FSH and pituitary LH and FSH did not differ among the three groups. These data are consistent with a possible role for progesterone in the placental enlargement seen in alcohol-fed rats.