Interleukin-1 (IL-1) has been implicated as a participant in preterm labor that is induced by bacterial infection. Previously, we showed that serotonin-induced production of IL-1 by myometrial smooth muscle cells in vitro is also essential for the synthesis of interstitial collagenase. It is therefore likely that IL-1 production in uterine tissues has implications for both the normal physiology of involution and for the pathophysiological mechanisms of preterm labor. The objective of this study was to characterize the serotonin-induced production of IL-1 by myometrial cultures in vitro and to assess the production of IL-1 and its relationship to collagenase production in vivo during pregnancy and the postpartum period. Immunohistochemistry demonstrated IL-1 protein in the nuclei and cytoplasm of serotonin-treated myometrial cells. IL-1 levels were decreased by treatment with progesterone or IL-1-receptor antagonist but were unaffected by lipopolysaccharide. Western analysis of myometrium from pregnant rats showed low levels of IL-1 during midpregnancy with increased concentrations at days 21 and 22 and postpartum. IL-1 mRNA levels also increased from days 15 to 22. Levels of mRNA for IL-1 also increased, although to a lesser degree than IL-1. Both mRNAs decreased postpartum. Conversely, mRNA for interstitial collagenase was barely detectable at term but increased postpartum. Together, these data show that serotonin stimulates IL-1 production in vitro and indicate that normal myometrium from pregnant rats is an identifiable source of IL-1 during late pregnancy. The findings are consistent with the possibility that myometrial IL-1 participates in normal labor as well as the postpartum production of interstitial collagenase.