Using a peptide extraction procedure, reversed phase high performance liquid chromatography, and a radioimmunoassay that utilized an antibody raised specifically against human beta-casomorphin-8 (BC8), BC-immunoreactivity (BCIR) was detected in rostrocaudally increasing levels in nineteen microscopically distinct and functionally relevant areas of mesencephalon, pons cerebri, and medulla oblongata of eight infants. On the basis of the methodology used, it can be concluded, that the BCIR present in their brain stem was due to BC8 and/or to some of its congeners. Data in the literature together with those of this study indicate that beta-casomorphins could be transported by specific mechanisms from the blood into the brain stem and that they could play a role in the central regulation of various physiological phenomena.