Interleukin-1 (IL-1) is a potent inhibitor of Leydig cell function. We have reported that IL-1 inhibited hCG-induced cAMP and testosterone formation. In the present study we evaluated the effect of IL-1 on Leydig cell cholesterol side-chain cleavage cytochrome P450 (P450scc) mRNA levels. P450scc is the rate-limiting enzyme for Leydig cell steroidogenesis. Highly purified Leydig cells were prepared from adult Sprague-Dawley male rats (55-65 day-old) using the combination of elutriation and Percoll gradient. Purified Leydig cells were then cultured with or without IL-1 beta (1-100 ng/ml) and recombinant human monocyte-derived IL-1 receptor antagonist (250 ng/ml) for 24 h. hCG (10 ng/ml), 8-bromo-cAMP (0.1 mM), or 4 beta-phorbol 12 beta-myristate 13 alpha-acetate was then added, and cultures were continued for an additional 6 h. P450scc mRNA levels of Leydig cells were very low to undetectable after 24 h in culture and could be stimulated by the addition of either hCG (10 ng/ml) or 8-bromo-cAMP (0.1 mM), but the addition of 4 beta-phorbol 12 beta-myristate 13 alpha-acetate had no effect. P450scc mRNA levels increased as early as 2 h after the addition of hCG. Furthermore, cycloheximide (1 microgram/ml) markedly blocked hCG-induced P450scc mRNA expression. This indicates that synthesis of a labile new protein(s) is required for the induction of P450scc mRNA by hCG. IL-1 beta inhibited hCG-stimulated testosterone formation and P450scc mRNA expression in a dose-dependent manner. The inhibitory effects of IL-1 beta could be reversed by the concomitant addition of IL-1 receptor antagonist. Our results suggest that P450scc mRNA levels of Leydig cells are modulated by IL-1. This may be one mechanism that could explain the inhibitory effects of IL-1 on Leydig cell steroidogenesis.