The effect of heat stress on plasma prostacyclin and thromboxane A2 and serum estradiol, estriol and progesterone responses was studied in pregnant and non-pregnant women. Group I consisted of 15 healthy non-pregnant women, group II of 23 women 13-14 weeks pregnant, and group III of 23 women 36-37 weeks pregnant. Blood samples were taken before the heat stress, at the end of the stress (70 degrees C for 20 min) and 20 min and/or 45 min after the end of stress. The rectal temperature increased 0.3-0.4 degrees C. The concentration of progesterone did not change during the experiment but that of estradiol increased by 11% (P less than 0.005) in group II and by 10% (P less than 0.01) in group III after the end of the stress. Estriol increased only in group III (by 12%, P less than 0.005) after the end of the stress and the increase was higher as compared to group II (P less than 0.005). The metabolite of prostacyclin increased only in group III by 15% (P less than 0.05) during the heat stress. The metabolite of thromboxane A2 decreased in group II by 20% (P less than 0.005) at the end of the stress while there was no change in group III. The fetal heart rate reactivity remained unchanged and only few uterine contractions were recorded. The small changes found in the levels of prostanoids and placental steroids in response to heat stress do not seem to have any deleterious effects on fetal well-being. The slightly increased concentration of placental steroids may rather reflect changes in metabolism than an increase in uteroplacental blood flow.