In this immunohistochemical study, the ontogenic development of calcitonin-gene-related peptide (CGRP) in the rat thyroid was investigated and compared with that of calcitonin using the indirect-immunofluorescence method. Parafollicular cells with immunoreactivity to both CGRP and calcitonin first appeared at an early stage of gestation (days 17 and 18) in the central portion of the thyroid. Cells immunoreactive to CGRP and calcitonin had became numerous by gestational day 22. After postnatal day 7, CGRP- and calcitonin-immunoreactive (C-IR) cells increased rapidly both in number and in the intensity of their fluorescence. In 14- to 90-day old rats, many intensely immunoreactive cells were distributed in the central portion of the thyroid. The cells immunoreactive to CGRP and to calcitonin had an almost identical ontogenic appearance. In 14-day-old and adult rats, C-IR cells also exhibited CGRP immunostaining, suggesting that these cells simultaneously produce and store CGRP during ontogeny.