Cycloleucine, a non-metabolizable amino acid analogue produces status spongiosus in cerebral white matter of rats and mice as well as a distinctive lesion of astrocytes. Its mechanisms of action include competition with natural amino acids from transport across the blood-brain barrier leading to inhibition of entry of circulating amino acids into brain, interference with ribosomal RNA maturation, and blockage of transmethylation reactions, including the conversion of homocystine to methionine. Cycloleucine also affects the kidney, producing aminoaciduria. Electron microscopy of cerebral white matter reveals spongiform changes of myelin sheaths caused by separation of myelin lamellae along intraperiod lines and accumulation of whorls of filaments in astrocytes. The myelinopathy is dose related and its toxicity is cumulative due to its long half-life in animals. The findings are discussed with reference to other spongiform myelinopathies, including status spongiosus observed in homocystinuria and other aminoacidurias.