Biopsies of human normal adrenal medulla, adrenal pheochromocytoma, and chemodectoma were studied for the presence of basic fibroblast growth factor (bFGF). An immunoreactive M(r) 18,000 bFGF-like molecule was detected both in normal and neoplastic tissues. This molecule was identified as bFGF on the basis of its molecular weight, its affinity for heparin, and its capacity to induce plasminogen activator production in cultured endothelial GM 7373 cells. The levels of immunoreactive and biologically active bFGF appeared to be significantly higher in the extracts of adrenal pheochromocytoma and chemodectoma than in the extracts of normal adrenal medulla. bFGF immunostaining was detectable in the nuclei of chief (Type-I) cells and of endothelial cells both in normal adrenal medulla and in pheochromocytoma. Cytoplasmic bFGF positivity of endothelial cells was also observed in pheochromocytoma but not in normal tissue. The data are in keeping with the hypothesis that bFGF may exert autocrine and paracrine functions in the growth and neovascularization of human pheochromocytoma.