Cysteine dioxygenase activity was not detected in the liver of either fetal or 2-day-old rat. The enzyme activity of neonatal rat liver gradually increased between the 4th and 12th postnatal days and then sharply increased to reach the adult level by the 28th postnatal day. No cysteine dioxygenase activity was observed in the fetal liver at the 18th day of gestation from a dam injected with hydrocortisone. Hydrocortisone-mediated induction was first observed in the livers of 4-day-old rats. The duration of hydrocortisone-mediated induction was much longer in the livers of 4- to 12-day-old rats than in those of adult rats. The enzyme half-life in livers of 10- and 20-day-old rats was 6.4 and 3.7 h, respectively. The half-life of the enzyme in 20-day-old rat liver was increased to 6.2 h by L-cysteine injection.