A polyclonal affinity-purified antibody to the carboxyl-terminal end of the rat monocarboxylate transporter 1 (MCT1) was generated in chickens and used in immunocytochemical studies of brain tissue sections from adult and suckling rats. The antibody identified a 48-kDa band on immunoblots and stained tissue sections of heart, cecum, kidney, and skeletal muscle, consistent with the reported molecular mass and cellular expression for this transporter. In tissue sections from adult brains, the antibody labeled brain microvessel endothelial cells, ependymocytes, glial-limiting membranes, and neuropil. In brain sections from 3- to 14-day-old rats, microvessels were much more strongly labeled and neuropil was weakly labeled compared with sections from adults. Immunoelectron microscopy indicated that labeling was present on both luminal and abluminal endothelial cell plasma membranes. These results suggest that MCT1 may play an important role in the passage of lactate and other monocarboxylates across the blood-brain barrier and that suckling rats may be especially dependent on this transporter to supply energy substrates to the brain.