Cannulas were implanted in the lateral ventricles, cisterna magna and subarachnoid spaces of goats. Perfusion with synthetic cerebrospinal fluid (CSF) permitted study of exchange rates of test materials between CSF perfusion fluid and blood. Diodrast or phenolsulfonphthalein in low concentrations is removed from ventriculocisternal perfusate by a process of active transport resembling secretion by the proximal tubules of the kidney. The transfer maximum is 2–3 µg/min. and is reached at an inflow concentration of about 20 µg/ml. Active transport occurs from a volume of about 2 ml (total CSF volume is c 20 ml) located in the region of the 4th ventricle and cisterna magna. The choroid plexus of the 4th ventricle could be the site of active transport. After inhibition of active Diodrast transport, a passive component of transfer is revealed. Comparative studies with creatinine, fructose and inulin show that passive transfer takes place by diffusion as well as by absorption in bulk. Rates of passive transfer of these substances (per unit concentration difference) are comparable in magnitude with diffusion rates from the capillaries in 1 gm of skeletal muscle.