Harvey sarcoma virus induces a number of neuron-associated properties in a nerve growth factor-responsive cell line, PC12 (Noda, M., Ko, M., Ogura, A., Liu, D., Amano, T., Takano, T., and Ikawa, Y. (1985) Nature 318, 73-75). We investigated the mechanism of this phenomenon using PC12 sublines cotransfected with a plasmid containing v-Ha-ras gene under control of a hormone-responsive enhancer/promoter element of the mouse mammary tumor virus long terminal repeat (pM14-1) and a plasmid encoding G418 resistance (pSV2neo). Extent of the expression of neuron-associated properties in several cell clones after the addition of dexamethazone (DEX) seems to correlate well with the levels of the v-Ha-ras gene expression. After the induction of v-Ha-ras expression with DEX in these cell lines, sustained elevation of the levels of cAMP as well as of phosphatidylinositol (PtdIns) metabolites, inositol trisphosphate, and diacylglycerol, is observed. Physiological significance of this phenomenon is confirmed by the observation that dibutyryl cAMP and phorbol-12-myristate 13-acetate synergistically induces the expression of neuron-associated properties in PC12 cells. In control PC12 sublines transfected with pSV2neo alone, DEX shows no effects on their cell morphology and the levels of cAMP and the PtdIns metabolites, although these control cell lines are competent to the effects of dibutyryl cAMP and phorbol ester. The priming activity known to be associated with nerve growth factor is also observed with v-Ha-ras as well as with dibutyryl cAMP plus phorbol ester but not with dibutyryl cAMP or phorbol ester alone. The observations suggest that the role of v-Ha-ras gene product in this system may involved simultaneous activation of the two signaling pathways, those mediated by cAMP and by PtdIns turnover.