In order to evaluate blood brain barrier (BBB) integrity and intra-BBB IgG synthesis under CSF pathologic conditions, it is necessary to establish normal values in a control population. In this study an evaluation was made of the blood brain barrier (BBB) integrity and intra-BBB IgG synthesis in the central nervous system (CNS) of normal juvenile rhesus monkeys to establish these values. Parallel cisternal cerebral spinal fluid (CSF) and venous serum samples were collected from 12 healthy male rhesus monkeys. Normal values for CSF/serum ratios of IgG and of albumin were determined and incorporated in theTourtellotte formula for quantitation of intrathecally synthesized IgG per 24 hours. The effect of systemic mannitol administration on the BBB was also evaluated by the primate adapted formula and magnetic resonance imaging contrast enhanced studies.