Growth factors like NGF are known to increase the expression of PrP gene, a housekeeping gene which is responsible for susceptibility to transmissible spongiform encephalopathies. We evaluated in vitro the effect of recombinant human growth hormone (hGH) and one of its in vivo effectors, the insulin-like growth factor I (IGF-I), on PrP gene expression in PC12 cells. We observed a 30% increase of PrP mRNA level after 7 day treatment by hGH at 10 micrograms/ml and potentiation of NGF effect (reaching four times baseline expression as opposed to three times baseline with NGF alone). IGF-I induced a dose-dependent increase of PrP mRNA up to twice baseline at a dose of 100 ng/ml and had an additive effect with NGF at 10 ng/ml. These preliminary results indicate that growth promoting factors may play a role in the PrP gene regulation within neuron-like cells.