The distribution of immunoreactive α-melanocyte stimulating hormone (α-MSHI) in the rat lower brain stem was examined by indirect immunofluorescence or peroxidase-anti-peroxidase immunohistochemical method using an antiserum against synthetic α-MSH. The results confirmed the presence of α-MSHI fibers in the midbrain central gray matter and parabrachial area, and demonstrated a much more extensive distribution of these fibers in various parts of the lower brain stem areas previously thought not contain α-MSHI fibers. In addition, the commissural nucleus was identified as a new α-MSHI neurons-containing site. No α-MSHI neurons were seen in other regions of the rat lower brain stem.