6-week-old, female albino rats were fed one of three diets containing 5, 10 and 20% casein for a period of 15 days. Rats fed the low protein diet (5% casein) lost weight (6.3 +/- 0.7 g/week), whereas those on the two higher protein diets gained weight. The concentrations of protein and free amino nitrogen in the brain were significantly lower in those on the low protein diet (5% casein) compared to those on the high protein diet (20% casein). The activities of brain enzymes, glutamine synthetase, glutamine transferase, glutaminase I, glutaminase II and glutamate decarboxylase, and the concentrations of free amino acids, aspartic acid, glutamic acid, glutamine, alanine and GABA were also lower. The prospect for nutritional rehabilitation of rats fed the low protein diet appeared to be excellent and was illustrated by the reversal of the above changes after 15 days on the high protein diet. The diet containing 10% casein was sufficient for the normal production of enzymes and free amino acids related to glutamate metabolism.