The distribution of enkephalins in human carotid bodies showing cellular proliferation and chronic glomitis. Human carotid bodies obtained at necropsy that showed prominence of either the sustentacular cell or the dark variant of chief cell or chronic carotid glomitis were studied by an immunogold labeling technique. The peptides methionine and leucine enkephalin had a similar distribution to that found in the normal human carotid body. They were localized prominently and predominantly in the dark and progenitor variants of chief (type I) cells. The sustentacular (type II) cells showed no immunoreactivity for the enkephalins. Cell counts on immunolabeled chief cells in cases of sustentacular cell hyperplasia and chronic carotid glomitis were found to be at the lower end of the normal range, whereas those in dark cell prominence occurred nearer the upper limit. 