The distribution of the enzyme butyrylcholinesterase (BChE) in dog brain cortex and cerebellum was studied by light and electron microscopy using a Hatchett's brown technique. Staining was found in neuron cell bodies and processes, the white matter of the cerebellum, and in capillaries and arterioles. Electron microscopy indicated that the enzyme activity associated with vessels was present in pericytes. Reaction product was found at the cell membrane, the intermembranous space of the nuclear envelope, and the Golgi complex of these cells. The finding of BChE in canine brain pericytes and its absence in endothelium does not support the idea that this enzyme is important in blood-brain barrier function. The pericyte in dog brain may be a site of synthesis of this enzyme and is, in this respect, similar to the endothelial cell of rat brain.