We have studied tyrosine aminotransferase in the liver of adult and old rats. Thermostability and trypsin action were not modified, and no charge differences have been found by isoelectric focusing between ‘adult’ and ‘old’ enzymes. Inducibility by glucocorticoids was increased in vivo in these 27- to 31-month-old rats, but not in vitro, in cultured hepatocytes. Moreover, we have shown the absence of ‘cross-reacting material’ for tyrosine aminotransferase in senescent rat livers. The rapid turnover of this enzyme may explain the apparent absence of alterations d