Using the immunohistochemical streptavidin-biotin-peroxidase complex tech- nique, the ultrastructure and localization of calcitonin gene-related peptide(CGRP)- immunoreactive nerve fibres were investigated in the stellate ganglion of guinea pig. The immunoreactive fibres were nonmyelinated and composed of nonvaricose and varicose parts. The latter was again divided into varicosities and intervari- cose segments. The immunoreactive varicosities contained many small clear vesi- cles, large cored vesicles and a few mitochondria, etc. They formed asymmetric or symmetric axodendritic and axosomatic synaptic contacts with nonreactive neuronal dendrites and somata as well as unspecialized appositions to them. The varicosities and intervaricose segments were partly exposed to the interstitial space. The CGRP-immunoreactivities were mainly localized in the axoplasmic ransport granules of the nonvaricose part, cytoplasma of the intervaricose segment, and the dense cores of the large cored vesicles of the varicosities. These obser- vations suggested that the CGRP could be also released by nonsynaptic diffusion in addition to synaptic release. The origins of the CGRP-immunoreactive fibres in the stellate ganglion of guinea pig were discussed.