The innervation of the cardiac ganglia of rats was investigated by light and electron microscopies. The same specimens were studied by histochemical methods. The cardiac ganglia were distributed densely around the pulmonary veins and consisted of nerve cells, satellite cells, nerve fibers, Schwann cells and connective tissue containing many fat cells. Most preganglionic nerve fibers ended in the dendrites of nerve cells, which were surrounded by Schwann cells. A few fibers invaginated and ended in the nerve cell body. Most nerve endings contained only non-cored vesicles (50nm), but a few endings contained large dense core vesicles (100nm) as well as non-cored vesicles. Under light microscopy, the acetylcholinesterase (AChE) reaction was strongly positive in all nerve cells of all cardiac ganglia. By electoron microscopy, the reaction site was found to be the narrow space between axon and nerve ending membranes and Schwann cells.These ganglia were investigated by the Falck-Hillarp method by fluorescence microscopy, and we confirmed that all ganglia did not contain sympathetic nerve endings. However there were small cells (10 microns in diameter) which showed intense fluorescence. These cells contained osmiophillic granules (150-300nm) and showed features similar to small intensely fluorescent cells (SIF cells). Around these cells, fenestrated capillaries were observed. From these results, it was confirmed that the cardiac ganglia of rats were innervated by parasympathetic nerves and that SIF cells were present.