The single pass clearance of glycine from the cerebral capillary bed was measured in the rabbit with the indicator diffusion method. It was shown that glycine was extracted to the same degree as fructose (ca..07). The concentration of glycine injected into the carotid artery was varied some 700-fold without significantly affecting the clearance of this amino acid in transit through the brain. Cross inhibition of the unidirectional movement of glycine was not demonstrable. These present results and most previous studies suggest that this putative neurotransmitter does not cross the blood-brain barrier by a carrier mediated process.