The distribution of several neuropeptides in subepicardial atrial ganglia was examined in the rat heart by using immunohistochemical techniques. Some of the ganglion cells in the right atrium exhibited neuropeptide Y-immunoreactivity while no immunostaining was found with antisera against atrial natriuretic polypeptide (ANF), substance P, vasoactive intestinal polypeptide (VIP), and calcitonin gene-related peptide (CGRP). Nerve fibers in and around the ganglia showed neuropeptide Y, calcitonin gene-related peptide and vasoactive intestinal polypeptide immunoreactivities. Some of these immunoreactive fibers (substance P, calcitonin gene-related peptide, neuropeptide Y) formed relatively dense networks around blood vessels and capillaries. Atrial natriuretic polypeptide-like immunoreactivity which was present in atrial myocytes failed to show up in any neuronal elements in the rat heart.