Liver tyrosine transaminase activity is low during the day when the rats are mostly asleep and high during the night when they are awake. When wakefulness was imposed for 8 hr during daylight on the day of the experiment and the rats were allowed to sleep for the following 3 hr during darkness, the tyrosine transaminase activity became high during the day and low at night. That this reversal in enzyme activity is not mediated by the pituitary-adrenal axis is demonstrated by the fact that in adrenalectomized rats tyrosine transaminase activity increased during the day in the sleep deprived rats. However, in these rats the enzyme activity did not become low in the sleep-deprived-sleeping condition. Changes in tryptophan pyrrolase activity during sleep deprivation were demonstrated to be mediated by the pituitary-adrenal axis.