A single subcutaneous injection of L-cysteine (1.2 mg/g body weight) to rats 4 days after birth was followed by atrophy of the brain which was well developed 27--32 days after the injection. It was apparent that the lesioned animals could be divided into two groups (type 1 and 2) on account of the degree of brain atrophy. In type 1, which was observed in 80% of the animals, the body weight was unchanged, but the total brain weight was reduced by about 20%. The brain structures most affected were cerebral cortex, hippocampus and thalamus, each having a 30--40% reduction in wet weight. The atrophy of the posterior part of cortex was particularly pronounced in this type of lesion. In type 2 lesion, which appeared in 10% of the survivors, the atrophy was much more severe. There was a 50% reduction in wet weight of brain and in body weight. The most prominent finding was the atrophy of the whole cortex and the hippocampus which were reduced by 80 and 60% of wet weights respectively. In this type of lesion significant morphological changes were observed in several brain regions such as caudato-putamen, thalamus, pons, medulla oblongata, spinal cord and cerebellum.