The presence of muscle spindles in the tongue of the rhesus monkey and their absence in the cat's tongue, raises the question of whether this difference alters the structural and functional composition of, in particular, the rhesus monkey's hypoglossal nerve as compared to the same nerve in the cat. Using both random and complete sampling techniques, calibre spectra analyses were carried out on the following regions of the rhesus monkey's hypoglossal nerve: The medial and lateral end branches, and regions of the main trunk proximal and distal to the point where the ramus descendens is given off. In addition, the calibre spectrum of the lingual nerve was determined. The results suggested that neither the lingual nerve nor the proximal region of the hypoglossal nerve contained enough fibers of Group I diameter to justify the statement that they contained spindle afferents. Distal to the point where the cervical contribution joins the hypoglossal nerve, however, the calibre spectrum exhibited a marked shift towards a closer resemblance to the spectra of limb muscle nerves in terms of the proportion of Group I diameter fibers. These findings suggested that although lingual spindle afferent fibers are present in the distal portion of the monkey's twelfth nerve, they may not reach the central nervous system in this nerve.