1.1. The greatest increase in hepatic cysteine oxidase activity was observed in intact rats which received cysteine, but the increase in tyrosine transaminase activity mediated by cysteine was only one-half of that produced with nicotinamide or hydrocortisone.2.2. The decline of the elevated enzymic activity of either cysteine oxidase or tyrosine transaminase was delayed when these enzymes were induced by nicotinamide in intact rats.3.3. An additive increase in cystein oxidase activity in the intact rat was obtained by the simultaneous injection of nicotinamide with cysteine and of nicotinamide with hydrocortisone but not of cysteine with hydrocortisone.4.4. Hepatic cysteine oxidase and tyrosine transaminase activities in adrenalectomized rats were significantly increased after injection of hydrocortisone but not by cystein. Nicotinamide did produce a marked increase in tyrosine transaminase activity in adrenalectomized rats, while cysteine oxidase activity did not increase with either nicotinamide or cysteine. The hydrocortisone-mediated induction of hepatic cysteine oxidase activity was markedly enhanced by simultaneous injection of cysteine, but in the case of hepatic tyrosine transaminase activity, the hydrocortisone-mediated induction was not affected by cysteine injection.5.5. Cysteine-mediated induction of hepatic cysteine oxidase was partially inhibited by the simultaneous injection of cycloheximide, whereas hydrocortisone- and nicotinamide-mediated inductions were completely inhibited by cycloheximide.