The pattern of innervation in the frog's heart shown by silver staining has been compared with the results of histochemical methods. The muscle innervation is a series of dense networks of nerve bundles. These have been divided into four groups according to their size and distribution. The silver methods show fewer fibres than the histochemical methods but all the methods show the same distribution of fibres. The ganglion cells in the vagal, dorsal root and sympathetic ganglia, and in the vagosympathetic trunks and heart, have been examined with silver staining and histochemical methods. It has been concluded that the vagal ganglion cells are probably sensory; that the sympathetic ganglion cells are confined to the sympathetic chain; and that all the cells in the vagosympathetic trunks and heart are parasympathetic motor cells. Section of the vagosympathetic trunks causes loss of all fluorescent nerve fibres from the heart, while at the same time there are no changes in the distribution of acetylcholinesterase-containing fibres in the muscle. It is concluded that the fluorescence and acetylcholinesterase methods show respectively the sympathetic and parasympathetic postganglionic fibres. There is no evidence from this work to support the claims that some recently described and extensive plexuses of fibres on the muscle are in fact nervous in origin.