1. The protein composition and concentrations of total protein in cerebrospinal fluid (CSF) and plasma of opossum (Monodelphis domestica) from birth until adulthood have been estimated. 2. Total protein in CSF increased from birth to a peak concentration between 5 and 10 days (500 mg/100 ml) after which it declined rapidly. 3. Total protein in plasma started at a low level at birth (below 400 mg/100 ml) and increased progressively to the adult value (8500 mg/100 ml). 4. Proteins identified in CSF and plasma were: albumin, alpha 1 antitrypsin, transferrin, alpha 2 macroglobulin, lipoproteins and immunoglobulin G. A fetal protein, probably alpha-fetoprotein, has been identified. 5. The only plasma proteins found within the brain, even in the very immature newborn of this species, appeared to have an intracellular rather than an extracellular distribution. This indicates that the blood-brain barrier is present very early in development and that there is also a CSF-brain barrier that appears to exclude CSF protein from brain extracellular space.