Nucleotide sequence analysis of part of the human calcitonin (hCT) gene revealed the presence of a region with 90% homology to the region in the corresponding rat gene encoding calcitonin gene related peptide (CGRP). Analysis of RNA isolated from human medullary thyroid carcinoma (MTC) indicated that in these tumors mRNA hybridizing to hCT specific probe, mRNA hybridizing to hCGRP specific probe and larger RNA species hybridizing to both hCT and hCGRP probes occur. These results suggest that alternative splicing of the primary transcripts of the human CT gene may result in hCT or hCGRP mRNA.