The rates of synthesis and degradation of arginosuccinate synthetase in rat liver under various dietary conditions were determined. The relative rate of the enzyme synthesis in the livers of rats fed on 70% casein diet was 4.0 times greater than that for rats fed on 5% casein diet. The rate constants of degradation (Kd of argininosuccinate synthetase were estimated to be 0.15 and 0.16 day-1 under 70% and 5% casein feeding, respectively. When the dietary conditions were changed acutely from 70% to 5% casein diet or vice versa, the rates of the enzyme synthesis decreased or increased, respectively, and the rates of enzyme degradation were also affected. The change from 5% to 70% casein diet caused a transient decrease in the rate of degradation. After the enzyme activity had achieved a new steady-state level, the enzyme degradation proceeded at the normal steady rate. On the other hand, the change from 70% to 5% casein diet caused a transient increase in the rate of degradation. Thus, the only factor regulating the amount of enzyme in rat liver is the rate of enzyme synthesis under the steady-state conditions. However, the rates of both enzyme synthesis and degradation are involved in the regulation of the amount of enzyme during dietary transition.