In adult rats, exposure to a novel environment, such as a simulated open field, elicits an increase in body core temperature. We have recently shown that this response is attenuated in midpregnancy and abolished at term of pregnancy in rats. We postulated that this gestation-dependent response resulted from alterations in the hypothalamic-pituitary-adrenal axis. To test this hypothesis, we measured the effects of pregnancy on renin, corticosterone, and arginine vasopressin (AVP) responses to exposure to either a simulated open field (30 or 120 min) or to the home cage (30 or 120 min) in rats. Pregnancy increased renin and corticosterone levels but not plasma AVP levels. Exposure to an open field decreased renin and increased plasma AVP levels in nonpregnant rats and on days 15 and 20 of gestation in pregnant rats, compared with home cage responses. Serum corticosterone levels were elevated after exposure to an open field in nonpregnant and pregnant rats, compared with home cage rats, the effect being more prolonged on day 20 of gestation. These observations provide new information on endocrine changes during pregnancy in rats and may help to explain the attenuated stress-induced hyperthermic response to exposure to a novel environment seen near term of pregnancy.