The cytokine interleukin-1 (IL-1) is released by a variety of cells in response to infection or injury. Il-1 produces several neuroendocrine and behavioral effects, including a suppression of reproductive functions and goal-directed behaviors. The present study examined the effect of IL-1 on sexual behavior in male and female rats. The following behavioral tests were employed: preference for a sexually appropriate partner, preceptive (soliciting) behavior, the lordosis quotient (sexual receptivity), and mating performance. Peripheral tip) IL-1β, 2 or 10 μg/kg, injected 2 h before testing, significantly suppressed preceptive behavior and sexual receptivity in intact, normally cycling females. In ovariectomized rats treated with ovarian hormones, IL-1β (2 or to μg/kg) significantly decreased the preference for a sexually active male partner and suppressed preceptive behavior and sexual receptivity. These effects were evident 2, but not 4 or 6, h after IL-1β administration. Intracerebroventricular administration of IL-1β (10 ng/rat) also suppressed the preference for a male partner and preceptive behavior in normally cycling females. Similar doses of IL-1β had no suppressive effect on any aspect of male sexual behavior, and the highest dose even increased the preference for a receptive female partner. In contrast to the gender-specific effects on sexual behavior, the suppressive effects of IL-1β on activity in the open-field rest were comparable in male and female rats. The inhibition of female sexual behavior by IL-1 may be adaptive, in that it prevents conception while the animal is sick, thus reducing the risk of spontaneous abortion or abnormal development.