Summary ― The activity levels of hepatic and renal D -amino acid oxidase (EC 1.4.3.3), a key enzyme for D -amino acid utilization in mammals, were determined in growing rats after a 10 day period of protein undernutrition and subsequent refeeding. Reducing the protein intake for 10 days (3% casein diet) resulted in an 8% loss of the animals' mean body weight and a 50% decrease in the mean size of the liver and kidney as compared to the control animals fed on a 22% casein diet during the same period. When the undernourished rats were refed with the normal protein diet, their weight increased about four fold as compared with that of the control animals, and after a ten day period of refeeding, the lost body and tissue weights were completely recovered. As far as the specific activity of D -amino acid oxidase was concerned, a 44% reduction took place in the liver of rats subjected to protein undernutrition for 10 days. During the period of refeeding, however, the enzyme activity level increased slowly in comparison with the overall hepatic protein level, since its specific activity on day 10 was still 28% below that of the control rats. In sharp contrast, no significant change in the kidney enzyme level was observed throughout these nutritional manipulations. This