Although protein requirements have been shown to decrease in rats with age, very few studies have dealth with old animals. In addition, a model of protein deficiency in aged rats has not been established. Nineteen-month-old male Sprague-Dawley rats were fed a 25 (control), 5, or 3.5% casein diet for 1 week ad libitum. Liver protein and RNA content were reduced in the 3.5% casein group when compared to controls. No differences were observed between the control and the 5% casein group. Also, no differences were observed in incorporation of [14C]orotic acid into liver nuclear RNA between these two groups. However, this variable was significantly higher in the 3.5% casein group when compared to control. When comparing the results of this study with our previous studies in young adult rats, the 3.5% casein diet produced biochemical changes in liver that are qualitatively and quantitatively similar to the ones observed in young adult rats fed a 6% casein diet ad libitum, which allows a more direct comparison of the effect of protein deficiency on the aged rat.