Comparison between the positive N balance data and the actual N deposition in the same adult animals was made. In experiment 1, 24 mature 9-month old male rats were divided into three groups. One group was killed on day zero and served as the control. The two other groups were fed 25 or 49% casein diet for 8 weeks. Body weight was kept relatively constant by regulating the dietary intake. The N balance study (including body hair N loss) was conducted thrice intermittently, and body N was determined at the end of the experiment. It was found that the mean total-body N of the 49% casein groups was 967 mg higher than the 25% casein groups and 885 mg higher than the day-zero control group (P less than 0.01). The calculated N retention during the experimental period was 1876 mg for the 49% casein group. In experiment 2, a similar study was conducted except that the 25 and 49% casein diets were fed ad libitum, and body weight gain was allowed. The total-body N values of both test groups at the end of the feeding period were very close to each other, but significantly higher than that of the day-zero control group (P less than 0.01).