Horses sweat, men may perspire, but ladies merely glow. Quite how brightly they glisten can depend on their contraceptive pill, new research demonstrates. Women with damp armpits are more likely to be taking contraceptives that contain a combination of oestrogen and progesterone, while those swallowing only progesterone remain dry.

That sex hormones affect women's temperature regulation system is well known. Temperature fluctuates during the normal menstrual cycle as natural hormone levels rise and fall -- progesterone nudges the temperature up and oestrogen brings it down.

Oestrogen also reduces the temperature threshold for sweating and vasodilation -- the widening of capillaries just under the skin. The hormone will make exercising women red-faced and sweaty earlier, in other words.

But until now it has not been clear what effect a combination of the two hormones has on temperature and sweating. "I wanted to see if oestrogen would modify the temperature effects of progesterone," says Nina Stachenfeld, a physiologist at Yale University School of Medicine, Connecticut. Eight volunteers, a fleet of exercise bicycles and a few packets of contraceptives helped her to find out.

Each volunteer (none of whom had used oral contraceptives before) took one of two widely prescribed pills for four weeks before the experiment. Both pills prevent conception by halting egg development -- the 'combined' pill with a combination of synthetic oestrogen and progesterone, and the 'mini' pill with artificial progesterone alone.

Exercising women taking the combination pill broke into a sweat at an average of 37.3 degrees C, while those on progesterone-only pills stayed sweatless until their body temperature reached 38.1 degrees C. It may not sound much, but Stachenfeld was surprised. "That's an enormous difference," she says.

The progesterone pill also raised body temperature by an average 0.6 degrees C compared with the combined pill, which had no effect on either sweating or temperature. Stachenfeld and her colleagues report their results in the Journal of Applied Physiology1.

How sex hormones alter women's temperature regulation is yet to be determined, though researchers are getting closer to finding out. They believe the hormones bind to specific brain neurons affecting temperature regulation.

Anthony Milton, who studies fevers at Cambridge University, UK, is not concerned that progesterone raises temperature. "I don't know of any long term studies but I wouldn't have thought it would pose any threat to healthy young women," he says. "Fever in itself doesn't do you any harm."

A single degree C rise in temperature does raise heart rate by about 10 beats per minute, but women are screened for possible heart problems before receiving the pill. "The reaction to rising temperature may be delayed but the body will eventually respond and bring it back down," Milton adds.

If not, the results can be tragic. A memorial marks the spot where the British cyclist Tommy Simpson died climbing the 6,000-ft high Mt Ventoux in the 1967 Tour de France. Traces of amphetamines were found in his body -- the drug had stopped Simpson sweating until he overheated. "His body just blew up," Milton says.

Higher temperature, bringing possible increased risk of heart problems and high blood pressure is not the only side effect the pill can cause. Many women report nausea, even depression, mostly during the first few months as their bodies adapt to the flood of hormones. Progesterone's effect on temperature regulation is almost certainly similarly short-lived, Stachenfeld says.