We examined c-fos and c-myc expressions in pheochromocytoma tissues from six patients. All samples contained c-fos and c-myc transcripts, whereas mRNA from bovine adrenal medulla, as a control, did not contain these transcripts at detectable levels. Southern blot analysis revealed no amplification and no rearrangement of c-fos and c-myc genes. We also examined the gene expression of insulin-like growth factor-II (IGF-II), a mitogen for rat pheochromocytoma cells exerted by autocrine or paracrine fashion. All samples from the pheochromocytomas contained IGF-II transcripts as well as c-fos and c-myc transcripts. The constitutive expressions of c-fos and c-myc genes may be interpreted to mean that pheochromocytoma is in a state of growth stimulation in vivo by growth factors, including IGF-II.