The subclavius muscles of Wistar rats were dissected, and the morphological features and innervation of this muscle were macroscopically studied. The subclavius muscles of rats were found to be innervated dually by the dorsal and ventral subclavius nerves. The dorsal subclavius nerve was found to arise, in conjunction with the suprascapular and upper subscapular nerves, from the dorsal division of the superior trunk of the brachial plexus. The ventral subclavius, which was observed for the first time, was discovered to issue, together with the pectoral and the accessory phrenic nerves, from the superior and middle trunks of the brachial plexus. In rats, the appearance of the accessory phrenic nerve seemed to be a almost consistent phenomenon. From the close anatomical relationship of the dorsal and ventral subclavius nerves with the neighbouring nerves, it could be speculated that the subclavius muscle might develop from an anlage of the hypobranchial musculature near and/or in the junctional region between the hypobranchial and the pectoral regions of the body trunk. The region might, phylogenetically and ontogenetically, concomitantly with the development of the heart and lungs, undergo remarkable changes, to which variations of this muscle and its innervation could be attributed.