In this study,primary serum-free cultured rat granulosa cells (rGCs) were used as a cellular model to investigate the effects of fenvalerate on progesterone production. Various concentrations (0,1,5,25,125and625μM) of fenvalerate were added to the cell cultures for 24h.rGCs were stimulated by compounds such as follicle-stimulating hormone (FSH), 8-bromo-cAMP or 22(R)-hydroxycholesterol (22R-HC) Progesterone production and intracellular cAMP content were measured in control and treated groups.Expression of P450 side chain cleavage enzyme (P450scc) and steroidogenic acute regulatory protein(StAR)were monitored by real-time PCR and Western blotting.Results showed that fenvalerate inhibited basal progesterone production in rGCs in the absence of stimulators.This inhibition was stronger in the presence of FSH and was not fully reversed by 8-bromo-cAMP or 22R-HC.The increase of cAMP content,stimulated by FSH, was inhibited by fenvalerate implicating that the intracellular cAMP-dependent signal pathway was involved.Fenvalerate reduced mRNA and protein expression of P450scc.These results suggested that multi-site inhibition of progesterone production by fenvalerate including a cAMP-dependent protein kinase pathway and reduction on P450scc gene expression and/or its enzymatic activity in rGCs.