The accumulation of the myelin proteolipid protein during rat brain development was studied in the spinal cord, lower brain stem (pons and medulla), cerebellum, and upper brain stem. Proteolipid accumulation was assessed on electroblots of total membrane samples from each region between 2 and 18 days of age. Proteolipid could be detected in rat spinal cord as early as 2 days of age, in lower brain stem and cerebellum by 4 days of age and in upper brain stem by 10 days of age. The time course of myelin basic protein accumulation was essentially the same as that of the proteolipid in each of the brain regions.