Evidence of a direct interaction between sympathetic and parasympathetic elements in a cardiac parasympathetic ganglion is presented in this study. Experiments were done using acutely dissected or organ cultured parasympathetic cardiac ganglion preparations from Necturus maculosus (mudpuppy). The glyoxylic acid-induced fluorescence technique was used to visualize catecholamine-containing cells and fibers. Numerous long brightly fluorescent varicose fibers form a complex network over clusters of parasympathetic ganglion cells and strands of cardiac muscle. In addition to these fibers, there are numerous small brightly fluorescent interneurons (SIF cells) interspersed between individual parasympathetic ganglion cells. Long fibers and processes from the interneurons join to form bundles which arborize over groups of parasympathetic cells. In peripherally located smaller groups of ganglion cells there are no interneurons, but some of these parasympathetic cells appear to receive innervation from the long continuous fluorescent axons. Two experimental procedures were applied to support the conclusion that these long fibers were indeed sympathetic postganglionic axons: explants of cardiac ganglia were maintained for varying times to produce degeneration of any severed axons: chemical sympathectomy was produced by injection of 6-hydroxydopamine. The intrinsic SIF cells were apparently unaffected by both procedures. After 8 days in culture or after 6-OH dopamine treatment, all of the long continuous brightly fluorescent fibers, which normally intermingle with clusters of ganglion cells or innervate cardiac muscle, were absent. This indicates their extra-ganglionic origin. All of the isolated groups of parasympathetic ganglion cells not containing SIF cells were totally devoid of any catecholamine-containing fibers.