Body mass and sex as determining factors in the development of fever in rats.

1. We have investigated the effect of a single I.P. injection (20 microgram/kg) of bacterial endotoxin on rectal temperature in rats of both sexes and from a wide range of body mass. 2. In male rats, endotoxin produced a monophasic or a biphasic rise in temperature, or a monophasic fall. 3. The extent of the rise in rectal temperature in male rats is related to body mass. There is a statistically significant correlation between body mass and the mean change in temperature measured over 100 min after injection of endotoxin. 4. In the female rats, endotoxin produced a monophasic fall in rectal temperature. The extent of the fall was not significantly correlated with body mass. 5. We suggest that the effect of endotoxin in male rats is determined by the physical relationship between body mass and surface area, because larger animals developed greater fevers. 6. The difference between the effect of endotoxin in male and female rats may have a physiological explanation and may involve differences in susceptibility to cutaneous vasodilation occurring immediately after injection of endotoxin.