The effect of short-term heat stress on rectal and skin temperatures was measured in three groups: group I consisted of 15 non-pregnant, healthy women; group II of 23 women 13-14 weeks pregnant; and group III of 23 women 36-37 weeks pregnant. Rectal and skin temperatures were recorded every 5 min during a resting period (20 min, 21-23 degrees C) followed by thermal stress (20 min, 70 degrees C) and a recovery period (45 min, 21-23 degrees C). The rectal temperature decreased significantly in each group at the beginning of the stress. The total increase from pre-stress levels to the end of the heat stress was from 37.1 to 37.5 degrees C in group I, 37.2 to 37.6 degrees C in group II and 37.0 to 37.3 degrees C in group III (P less than 0.05). During the recovery period, the rectal temperature was significantly lower in the late-pregnancy group as compared to non-pregnant subjects. The skin temperature increased from the resting period to the end of the stress from 34.0 to 39.1 degrees C in group I, from 33.7 to 39.3 degrees C in group II and from 33.5 to 39.0 degrees C in group III (P less than 0.001). During the recovery period, the skin temperature declined significantly faster, and to lower levels in the late-pregnancy group as compared to both other groups. We conclude that the heat-dissipating ability is slightly enhanced during late pregnancy.