By immunohistochemistry, a large number of neurotensin immunoreactive nerve terminals are found in the kitten inferior olive of the medulla oblongata. They are present in the dorsal lamella of the principal olive, in the ventrolateral outgrowth and in the medial part of the caudal dorsal accessory olive. They are absent in the medial accessory olive. They disappear in the adult cat. Neurotensin immunoreactivity is absent in the developing rat inferior olive. This localization in the cat suggests a neuronal origin in the mesencephalon, mainly in the red nucleus. These results confirm our recent report on a transient large neurotensinergic innervation of the human developing principal olive.