It is generally believed that the elevated progesterone levels seen during the luteal phase of the menstrual cycle alter thermoregulatory function during exercise. We hypothesized that oral contraceptives (OCs) that contain synthetic progestogen would alter thermoregulation during exercise in a manner similar to that seen during the luteal phase. To test this hypothesis 10 healthy women currently taking OCs were recruited. Subjects performed a 1 h exercise bout in the heat (30 degrees C, 50% relative humidity; RH) at 60% of their maximum oxygen uptake on two occasions: once while on OCs and once during a control trial when the subjects were off OCs. Core temperature, skin temperatures, heart rate, skin blood flow and sweat rate were measured at rest and every 20 min during both exercise bouts. OCs significantly increased core temperature and heart rate over that seen during the control condition. Specifically, after 1 h of exercise, OCs caused a mean 0.3 degrees C and an 8-bpm increase in core temperature and heart rate respectively. Additionally, the differences in core temperature and heart rate during the two trials became more exaggerated as exercise duration increased, as evidenced by significant treatment-by-time interactions. These results suggest that OCs alter thermoregulatory and cardiovascular function during exercise in the heat. Furthermore, the changes in core temperature and heart rate seen during exercise while taking OCs are similar in direction and magnitude to those observed during the luteal phase of the menstrual cycle.