Rats of different ages (3 to 15-wk-old) were fed on a 25% casein diet for one week, and the nitrogen balance and liver serine dehydratase (SDH, EC 4.2.1.13) activity were then determined. The value for nitrogen balance decreased with the age of the rats, while the liver SDH activity increased. A statistical analysis showed clear inverse correlation between the two factors (R(2) = 0.7372, p < 0.01). This result suggests that SDH was induced by response to the amount of surplus amino acids from dietary protein taken beyond the body's requirement. The increase in SDH activity was accompanied by an increase in the level of SDH mRNA. Since the half-life of this mRNA did not change significantly, the induction was mainly controlled at the level of transcription. In addition, the induction seems not to be related to gluconeogenesis, since the mRNA levels of tyrosine aminotransferase (TAT) and phosphoenolpyruvate carboxykinase (PEPCK), other gluconeogenic enzymes, were not changed under these experimental conditions.