The trigeminal nerve has motor, intermediate and sensory roots in the dog. The motor root is composed of two bundles: the medial and ventral. The ventral motor root emerges from the pons, forming the common root with the sensory, and then is separated from it to run along with the medial motor root which separately exits from the pons as the ordinary motor root. The mesencephalic root fibers of the trigeminal nerve are investigated in the dog, using the myelin sheath degeneration technique. Degenerated fibers are observed numerously in the ventral motor root and moderately in the medial motor root, and are traced to the mandibular branch of the trigeminal nerve. Some degenerated fibers are observed in the sensory root and they seem to pass through the Gasserian ganglion to enter the ophthalamic and maxillary branches. No degenerated fibers are found in the intermediate root.