Animal cell growth factors stimulate expression of the proto-oncogenes c-myc and c-fos. The products of these genes seem to act as intracellular mediators of the mitogenic response to growth factors. Phosphatidyl inositol breakdown products function as cytoplasmic second messengers to induce transcription of c-myc and c-fos although they may not play an exclusive role in this regard. Post-transcriptional events may contribute to the modulation of c-myc gene expression. Following induction, the c-myc and c-fos mRNAs are selectively degraded within the cell.