Comprehension of the mesencephalic syndromes that affect oculomotor nerve fascicles requires a detailed knowledge of their relationship with the adjacent structures and the blood supply of the central midbrain region. This was the reasoning behind our study, which was performed in ten serially sectioned midbrains stained with cresyl violet and luxol fast blue, in three microdissected midbrains, and in two injected and cleared specimens. Three continuous groups of the intramesencephalic oculomotor nerve fascicles were distinguished: the caudal, intermediate and rostral. The caudal fascicles, which most likely innervate the superior rectus and the levator palpebrae superioris muscles, extend through the superior cerebellar peduncle just caudal to the red nucleus and close to the lateral lemniscus. The intermediate fascicles, devoted to the medial rectus and the inferior oblique muscles, always pass through the superior cerebellar peduncle, just medial to the caudal part of the red nucleus (60 %), and less frequently (40 %) through the nucleus itself. The rostral oculomotor fascicles, which terminate in the inferior rectus and sphincter pupillae muscles, course medial to the rostral part of the red nucleus. While the rostral and intermediate oculomotor fascicles are supplied only by the medial twigs of the paramedian mesencephalic perforating arteries, the caudal fascicles are also nourished by the lateral branches of the same perforating arteries. The data obtained form an important basis for the explanation of certain mesencephalic syndromes, and even anticipate some new syndromes not yet described in the literature.