This study was an examination of the effect of interleukin-1 alpha (IL-1 alpha) on prostaglandin (PG) production and cyclooxygenase (COX) activity in endometrial stromal cells isolated from uteri of ovariectomized rats sensitized for the decidual cell reaction. Treatment with IL-1 alpha (20 ng/ml) for 24 h caused approximately 13-, 2-, and 2-fold increases in PGE2, PGF2 alpha, and 13, 14-dihydro-15-keto-PGF2 alpha (PGFM) accumulation, respectively, in the medium. The IL-1 alpha-induced increase in PG accumulation was accompanied by a significant (6-fold) increase in COX activity in the cells. IL-1 alpha-induced increases in PGE2 accumulation and COX activity were greatly reduced or prevented by cycloheximide (CHX), actinomycin D, and dexamethasone (DEX). In addition to IL-1 alpha, epidermal growth factor (EGF, 40 ng/ml) also caused a significant increase in PGE2 accumulation in the medium and COX activity in the cells. The combined effects of IL-1 alpha and EGF on PGE2 accumulation and COX activity were additive. The results of this study provide evidence that IL-1 alpha stimulates PG production in endometrial stromal cells from uteri sensitized for the decidual cell reaction through a mechanism that involves an increase in COX activity.