Alternative splicing of rat calcitonin (CT) gene transcripts resulting in the production of calcitonin gene-related peptide (CGRP) in neural tissue and of CT in the thyroid has been proposed by Rosenfeld et al. (1983b). We have recognized CGRP- and CT-like peptides in extracts of the human brain, pituitary and thyroid using a combination of gel filtration analysis and HPLC, and specific RIAs. Immunoreactive CGRP was estimated in a heterologous RIA using 125I-labelled rat CGRP as ligand and antibodies raised to the rat hormone, and human CT in a homologous RIA. The levels of CGRP and CT are measured against synthetic rat CGRP and monomeric human CT, respectively, and expressed in ng and micrograms equivalents (eq). The content of immunoreactive CGRP of the neocortex (n = 3), the cerebellar cortex (n = 6), the periventricular mesencephalic region (n = 3) and the thyroid (n = 5) was similar (mean +/- SE, 0.79 +/- 0.16 ng eq/g wet tissue, 1.51 +/- 0.14 ng eq/g, 1.84 +/- 0.12 ng eq/g and 5.0 +/- 0.9 ng eq/g, respectively), whereas pituitary glands (n = 21) and medullary thyroid carcinomas (n = 6) contained higher levels (31.3 +/- 5.1 ng eq/g and 7.66 +/- 5.42 micrograms eq/g, respectively). Immunoreactive CT was lowest in the neocortex, cerebellar cortex and the periventricular mesencephalon (0.31 +/- 0.07 ng eq/g, 0.30 +/- 0.09 ng eq/g and 0.26 +/- 0.09 ng eq/g, respectively), followed by the pituitary and thyroid (2.77 +/- 0.62 ng eq/g and 146 +/- 26 ng eq/g, respectively), and was highest in medullary thyroid carcinoma tissue (680 +/- 372 micrograms eq/g).(ABSTRACT TRUNCATED AT 250 WORDS)