The effect of a moderate heat stress on cardiovascular responses was studied: 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. Heart rate, stroke volume, cardiac output, arterial blood pressure and peripheral vascular resistance were recorded every 5-10 minutes during a resting period (20 min, 21-23 degrees C) followed by heat stress (20 min, 70 degrees C, 15% relative humidity) and a recovery period (45 min, 21--23 degrees C). The rectal temperature increased 0.3-0.4 degrees C in each group during thermal stress. The heart rate before stress was highest in the advanced pregnancy group but increased almost identically in each group by 36--37 beats per minute during stress and approached starting values during recovery. There were no major changes in stroke volume during the experiment in any group nor were there any differences between the three groups. Arterial blood pressure did not change significantly in any group during the experiment; the differences between the groups were minimal. Peripheral vascular resistance began to fall at the start of the thermal stress and returned to prestress levels at the end of the recovery period. There were no differences between the groups in proportional changes of peripheral resistance. We conclude that pregnancy does not alter the cardiovascular responses to moderate thermal stress.