The adrenergic agonists and antagonists, norepinephrine, phenylephrine, iso‐proterenol, phentolamine, and propranolol, were administered to rats in different phases of the diurnal cycle, and their effects on tyrosine aminotransferase activity were studied. All substances tested, regardless whether being typicallyα‐orß‐adrenergic or whether being agonists or antagonists, elevated tyrosine aminotransferase during the minimum of enzyme activity. The effects at the enzyme maximum, however, were generally more or less depressive. The extent of the depressions by norepinephrine and by propranolol highly depended on the duration of treatment. The hepatic concentration of cyclic AMP did not exhibit a significant rhythmicity. The results do not favour the idea of an adrenergic control of the diurnal rhythm in tyrosine aminotransferase activity.