Plasma methionine-enkephalin-like and leucine-enkephalin-like immunoreactivity (met-enkephalin-LI and leu-enkephalin-LI, respectively) in six normal subjects and six patients with pheochromocytoma were determined. The contents of met-enkephalin-LI and leu-enkephalin-LI in two of six pheochromocytomas were 40- to 50-fold higher and those in the other four pheochromocytomas were less than those in normal human adrenal medulla. The former two patients showed high plasma met-enkephalin-LI and leu-enkephalin-LI levels. As plasma catecholamines levels returned to the normal range after extirpation of tumors, met-enkephalin-LI and leu-enkephalin-LI in plasma became undetectable in these patients. In contrast, neither met-enkephalin-LI nor leu-enkephalin-LI was detected in plasma from the latter four patients. Met-enkephalin-LI and leu-enkephalin-LI concentrations were higher in the adrenal vein than in the peripheral vein in three patients. Plasma met-enkephalin-LI and leu-enkephalin-LI increased concomitantly with catecholamines after glucagon stimulation and during a spontaneous attack in a patient with pheochromocytoma. Plasma met-enkephalin-LI changed in parallel with leu-enkephalin-LI in all cases. High performance liquid chromatography coupled with RIAs has shown that met-enkephalin and leu-enkephalin circulate in plasma from a patient with pheochromocytoma as intact pentapeptides. None of normal subjects showed detectable concentrations of met-enkephalin-LI and leu-enkephalin-LI in plasma (more than 5 pg/ml and 3 pg/ml, respectively). It is concluded that met-enkephalin and leu-enkephalin are released concomitantly with catecholamines from pheochromocytomas.