Abstract The objective of this study was to examine the effect of interleukin-1 β (IL-1) on progesterone (P) biosynthesis and the potential intermediary involvement of prostaglandin (PG) E and nitric oxide (NO) in P accumulation in PMSG/hCG-primed rat corpora luteal (CL) cell cultures. Exposure of primed CL cells to IL-1 (10 ng/ml) for 48 h resulted in a 65–86% reduction ( P<0.01) in P accumulation concurrent with a 2–3.4-fold increase in PGE content, a 70% increase in PGF 2 α content and a 1.9–3.3-fold increase in nitrite generation. These effects were abolished by the IL-1 receptor antagonist, suggesting specific IL-1 receptor-mediated effects. Indomethacin, a cyclooxygenase inhibitor, abolished PGE and PGF 2 α production and attenuated the basal (but not IL-1-stimulated) accumulation of P. N G-Nitro- l-arginine (NNLA), a competitive inhibitor of nitrite synthesis, slightly reduced basal P accumulation but had no effect on IL-1-induced suppression of P accumulation. NNLA reduced basal PGE accumulation and IL-1-stimulated PGE accumulation (55 and 61%, respectively). Transforming growth factor β1 (TGF- β1; 10 ng/ml) significantly attenuated the IL-1-stimulated PGE and NO production (61 and 42%, respectively), but did not affect the ability of IL-1 to suppress P accumulation. Thus, NO, PGF 2 α and PGE are not obligatory intermediaries of IL-1-mediated suppression of P accumulation in rat CL, but are involved in basal P biosynthesis and NO seems to have a regulatory role in the biosynthesis of PGE. The present observations suggest a pleiotropic response of PMSG/hCG-primed CL cells to IL-1, characterized by an independent suppression of P accumulation and a concomitant increase in NO, PGF 2 α and PGE generation. Since IL-1 attenuates P accumulation, these findings may imply a direct autocrine/paracrine function for IL-1 in the maintenance or the demise of rat CL.