The effects of short-term, moderate heat stress (20 min, 70 degrees C) on uterine contractility, fetal heart rate and fetal movements were investigated in 23 healthy subjects 36 to 37 weeks pregnant. The thermal stress caused minimal uterine contractility in five subjects but did not induce labor. Fetal heart rate increased from 146 beats per minute to a maximum of 157 beats per minute (P less than 0.05) by 10 min after termination of thermal stress. Fetal movements increased from 2.3 per minute to 4.9 per minute 35 minutes after heat stress (P less than 0.05). Throughout the experiment fetal heart rate variability remained normal but in four cases the tracings were temporarily non-reactive during or after the heat stress. The newborn infants were all in good condition at delivery which, generally, took place at the estimated time of delivery. The results suggest that moderate thermal stress does not induce regular uterine contractility nor is it harmful to the fetus at late pregnancy.