The daily rhythm in the activity of rat liver tyrosine aminotransferase was abolished after 3 weeks of constant illumination. The rhythm reappeared only partially in rats bilaterally adrenalectomized immediately after the 3-week period and returned to a cyclic lighting schedule. The failure of the enzyme rhythm to reappear fully did not seem to originate in dietary factors since food consumption and rate of growth were similar in the adrenalectomized and sham-operated control rats. Developmental studies showed that the adult rhythm was absent in rats up to 15 days old but present in 21-day-old rats. Although the rhythm appears to be generated in part by the rhythmic intake of dietary amino acids, a role for the endogenous glucocorticoids was suggested by the finding that physiological amounts of cortisol played a permissive role in the stimulation of enzyme activity by amino acids (casein hydrolysate) in adrenalectomized rats. These amounts of cortisol greatly stimulated enzyme activity in rats fed a protein-free diet. A regulatory role for glucocorticoids based on this permissive action is envisaged in the daily rhythm of tyrosine aminotransferase.