We studied the expression of the ret proto-oncogene (proto-ret) in human medullary thyroid carcinomas (MTCs) and pheochromocytomas of multiple endocrine neoplasia type 2A (MEN 2A) by Northern blot analysis. Expression of the normal-sized transcripts was detected in all 12 MTCs and in 6 of 8 pheochromocytomas. In situ localization of proto-ret mRNA revealed that the signal was confined to the cytoplasm of MTC cells. By Southern blot analysis neither amplification nor gross genetic changes of proto-ret were found in the tumors. Although no transcripts were detected in the normal portion of the thyroid from one MEN 2A patient, faint signals were detected in normal adrenal glands by Northern blot analysis, probably due to minor populations of C-cells and chromaffin cells in specimens from which MTC and pheochromocytoma might later develop. Proto-ret may play an important role in differentiation of a specific cell lineage from neuroectoderm, and it may be involved in development of MEN 2A tumors.