In three infants with nonketotic hyperglycinemia, glycine was increased three-to fourfold in plasma, 13- to 28-fold in lumbar spinal fluid, and was higher yet in ventricular fluid. Oral sodium benzoate lowered cerebrospinal fluid (CSF) glycine by >40%, but did not change the abnormal plasma: CSF ratio. An adult control, made hyperglycinemic with oral glycine, had a normal plasma: CSF ratio. Treatment of one patient with sodium benzoate from birth did not prevent mental retardation; the degree of brain stem depression was a function of CSF glycine in another patient. The persistance of glycine elevation in CSF, although therapy maintained normal concentration in plasma, appears to be caused by overproduction in brain and limitation of the high-capacity lumbar spinal reabsorptive mechanism. Treatment through lowering of CNS glycine by use of a ventricular shunt was explored.