It has been proposed that Sudden Infant Death Syndrome (SIDS) might occur as a consequence of a developmental deficit associated with the cardiorespiratory and arousal control centers located within the brainstem. In this study 1.1' dioctadecyl-3,3,3',3-tetramethylindocarbocyanine perchlorate (DiI) was used to investigate the trajectories of the glossopharyngeal and vagus nerves which carry essential afferent and efferent fiber tracts associated with cardiac and respiratory control and of the hypoglossal nerve which innervates the tongue, in SIDS (n = 14) and control (n = 7) infants. The postnatal development of the trajectories of these nerves was examined in non-SIDS brains and comparisons were then made with age-matched SIDS brains. The mean profile area of hypoglossal and dorsal motor neurons were also assessed. In controls, no major alterations were observed in the trajectories of axon bundles with increasing age (7 wk to 2 yr) in each of the nerves investigated although axon bundles appeared to increase in thickness with age. In SIDS cases (2 wk to 44 wk), the trajectories of the cranial nerves were not different from those seen in age-matched control cases. The mean profile area of hypoglossal and dorsal motor neurons was not significantly different between control and SIDS infants. We conclude that the DiI tracing technique can be used successfully to trace the pathways of cranial nerves in human infant fixed-tissue. Furthermore, if functional differences exist between SIDS and non-SIDS brains in the control of respiration, circulation, or arousal they do not appear to be related to markedly reduced or aberrant projections of the glossopharyngeal, vagus, or hypoglossal nerves.