N europeptide Y (NPY) levels are increased in plasma and tumors of patients with pheochromocytoma. The present study was designed to evaluate plasma and tissue NPY levels simultaneously as well as to study its release and expression in patients with either adrenal or extraadrenal pheochromocytomas. Plasma NPY levels were higher (P < 0.01) in patients with adrenal tumors than in matched normal subjects and patients with extraadrenal tumors. NPY levels were also higher (P < 0.05) in adrenal than in extraadrenal tumors. Bioactive NPY (1-36) was the predominant form in plasma and tumors of patients with adrenal pheochromocytomas. In contrast, patients with extraadrenal pheochromocytomas had an abundance of NPY fragments. NPY mRNA was abundant in 11 of 13 adrenal tumors but in only 1 of 6 extraadrenal tumors. Moreover, NPY was coreleased with NE with manipulation of adrenal but not extraadrenal tumors. These findings indicate that increased NPY gene expression in adrenal pheochromocytomas accounts for the greater biosynthesis and storage of NPY in these tumors and that increased release of NPY results in elevated plasma NPY. Factors regulating NPY gene expression in pheochromocytoma and the role of NPY in the clinical manifestations of the disease remain to be elucidated.