Leu-enkephalin, a potent, endogenous, opiate-like regulatory pcptide, is present in a subpopulation of normal adrenal medullary cells and in a spectrum of proliferative lesions of adrenal and extra-adrenal chromaffin cell origin. The presence, extent, and intensity of leusenkephalin-like immunoreactivity is variable in normal and pathological states. While areas of diffuse medullary hyperplasia consistently exhibited leu-enkephalin-like immunoreactivity, approximately 50% of hyper-plastic medullary nodules, pheochromocytomas, and paragangliomas were positively stained. Tumors of neuroblastic origin, on the other hand, did not contain leuenkephalin-like immunoreactivity. Variations in leuenkephalin-like immunoreactivity may be related to aberrations of feedback mechanisms, multicentric origins of lesions from chromaffin cells with or without the capacity for leu-enkephalin synthesis, or to a variety of other mechanisms, including defective innervation of hyperplastic and neoplastic chromaffin cells. The results of these studies indicate that leu-enkephalin-like immunoreactivity is a useful tissue marker for the demonstration of chromaffin cell hyperplasia and neoplasia and may also prove to be an important clinical marker for the assessment of chromaffin cell hyperfunction.