Endogenous release or exogenous administration of the cytokine Interleukin-1 (IL-1) produces several behavioral alterations, including suppression of locomotion and exploration. Because there are bidirectional interactions between IL-1 and the hypothalamic-pituitary-gonadal axis, we investigated possible differences between males and females in various phases of the estrus cycle in the behavioral effects of IL-1. In addition, we assessed the role of progesterone in mediating estrus cycle-dependent differences in these effects. Female rats in either the estrus or the non-estrus phase of their cycle and male rats were injected with either IL-1β (2 or 5 μg/kg) or saline. Activity in the open field test was measured 2 h later by counting the number of line crossings and rearings. In saline-injected rats, non-estrus females performed less line crossings than estrus females. IL-1 produced a significant dose-dependent reduction of line crossing in males and estrus females. In contrast, in non-estrus females the lower dose of IL-1 had no effect, and the effect of the higher dose was significantly smaller in non-estrus than in estrus females. The higher dose of IL-1 suppressed rearing in all three groups, but the effect of the lower dose on the number of rearings was significant only in estrus females. In a second experiment, ovariectomized females were injected with either progesterone (2 mg/rat) or oil, followed 2 h later by an injection of either IL-1β (2 μg/kg) or saline. Activity was measured continuously by a biotelemetric system. IL-1 reduced activity in progesterone-treated ovariectomized females but not in oil-injected controls. These findings suggest that changes in progesterone secretion along the estrus cycle modulate the behavioral responsiveness to IL-1 in female rats.