Recent studies of autonomic ganglia have shown that specific combinations of neuropeptides and other potential neurotransmitters distinguish different functional types of neurons. In the present paper the patterns of coexistence of neurochemicals in guinea-pig cardiac ganglion cells was examined, using multiple-labelling immunohistochemistry. Many neurons were found to contain somatostatin immunoreactivity with various combinations of immunoreactivity for dynorphin B, substance P, neuropeptide Y and nitric oxide synthase. There were several small populations of neurons without somatostatin immunoreactivity, which contained combinations of immunoreactivity for vasoactive intestinal peptide, neuropeptide Y, dynorphin B, substance P and nitric oxide synthase. Possible synaptic inputs to these populations of ganglion cells were identified using multiple-labelling immunohistochemistry combined with long-term organ culture. These experiments demonstrated that cardiac ganglia contain prominent pericellular baskets of varicose nerve terminals of sympathetic and sensory origin. In addition, populations of intrinsic intraganglionic nerve terminals were identified which were immunoreactive for vasoactive intestinal peptide, neuropeptide Y or both peptides. These terminals presumably originate from intrinsic neurons, with the same combinations of neuropeptides, located in other cardiac ganglia. These results have demonstrated that there are diverse populations of cardiac ganglion cells in the guinea-pig and that some of these neurons may act as interneurons within the intrinsic cardiac plexuses. Therefore it is highly likely that vagal transmission in the heart is modified by sympathetic, sensory and intrinsic neurons and that cardiac ganglia are complex integrators of convergent neuronal activity rather than simple relays.