Labor is preceded by cervical ripening through upregulation of interleukin (IL)-1β, IL-8, and increased prostaglandin synthesis via inducible type 2 cyclooxygenase (COX-2). Progesterone maintains myometrial quiescence during pregnancy. In this study, we examined the effects of IL-1β and progesterone on IL-8 and prostaglandin E₂ (PGE₂) synthesis and IL-8 and COX-2 mRNA and promoter activity in amnion cells and lower segment fibroblast (LSF) cells. In both cell types, progesterone had no effect on basal IL-8 or PGE₂ synthesis. In LSF cells, IL-1β significantly increased IL-8 and PGE₂ synthesis and COX-2 and IL-8 mRNA expression, but progesterone significantly attenuated these effects. In prelabor amnion cells, IL-1β also increased IL-8 and PGE₂ synthesis and both COX-2 and IL-8 mRNA and promoter expression; however, progesterone significantly attenuated these effects on IL-8 and PGE₂ synthesis and COX-2 expression. In postlabor amnion cells, IL-1β increased IL-8 and PGE₂ synthesis and COX-2 expression, but progesterone did not attenuate the effect of IL-1β upon IL-8 synthesis. Progesterone repression of IL-8 and COX-2 in LSF cells suggests that IL-8 and COX-2 have similar regulatory mechanisms in LSF cells and that progesterone may play a role in maintenance of cervical competence. The lack of effect of progesterone on IL-8 in postlabor cells may be the result of downregulation of the progesterone receptor during labor.