Calcitonin gene-related peptide (CGRP)-like immunoreactivity has been reported to occur in both varicose and non-varicose nerve fibers among and around principal ganglion cells in the celiac ganglion of rats. The present electron microscopic immunohistochemistry revealed a small number of myelinated nerve fibers immunoreactive for CGRP, although most of the immunoreactive fibers were unmyelinated. In the fetal development, CGRP-immunoreactive nerve fibers were first revealed at day 18 of gestation and thereafter gradually increased in number. Both the density and distribution of the immunoreactive fibers within the ganglion similar to those in adult rats were attained by postnatal day 14. This developmental change was compared with that of enkephalin-immunoreactive nerve fibers in the same ganglion. In electron microscopy, the immunoreactive fibers with varicosities were characterized by abundant small clear vesicles mixed with large granular vesicles. These formed numerous axo-dendritic and several axo-somatic synapses with the principal ganglion cells, whereby the immunoreactive fibers were presynaptic. The immunoreactive material was localized in the core of the large granular vesicles as well as in the axoplasm. On the other hand, the immunoreactive fibers without varicosities were characterized by neurofilaments, neurotubules and small mitochondria; small clear or large granular vesicles were rarely found in the immunoreactive fibers. They lay in no direct apposition to adjacent neuronal elements and were, therefore, regarded either as fibers passing through the ganglion or as preterminal portions of the CGRP-immunoreactive varicose terminals.