Serotonergic neurons in the caudal raphé are chemosensitive, are intimately associated with arteries, and are putative central chemoreceptors. In the rostral raphé, serotonergic neurons are also stimulated by acidosis in both tissue culture and brain slices (Wang et al, SFN Abstracts, 2002). Here we examined whether rostral raphé neurons have the same relationship with arteries as those in the caudal raphé. Adult rats were transcardially perfused with formalin and arteries were filled with FITC labeled gelatin. Serotonergic neurons were immunolabeled with TpOH antibodies and visualized with confocal microscopy. Median raphé neurons were closely associated with midline penetrating arteries. The complex shape of the dorsal raphé paralleled the turns and branches of deep arteries in that region. We also noted that the midbrain is the only region of the rat CNS that is supplied by both the anterior and posterior circulations. To ensure that these observations were relevant to chemosensitive cells, acidosis-sti...