Human fibroblasts treated with the antihypercholesterolaemic drug, lovastatin, displayed a diminished signaling response to epidermal growth factor (EGF), insulin and insulin-like growth factor I (IGF-I). Supplementing the culture medium with mevalonic acid restored the signaling response. Not all growth factor signaling pathways were impaired, however, as platelet-derived growth factor (PDGF-BB) and basic fibroblast growth factor (bFGF) responses were refractory to lovastatin treatment. These results suggest the involvement of product(s) of mevalonate metabolism (e.g., prenylated proteins such as p21ras or G proteins) in the signal transduction of EGF, insulin and IGF-I. The inhibition of cell growth by lovastatin may be caused by the inability of the cell to enter the S phase of the cell cycle due to obstruction of the signaling of progression factors.