Diurnal variations of the hepatic tyrosine aminotransferase (TAT) activity were studiedin rats treated with streptozotocin which induces hypoinsulinism resulting in diabetes. The rats had free access to a diet during one half of each day and were fasted during another half of the day in the entire period of the experiment. Clear diurnal variations of the TAT activity were observed in both normal and diabetic rats. The diurnal variations in the TAT activity were not primarily due to a light-dark cycle but in response to food intake : that is, the activity of TAT was high during the feeding period independently of a lightdark cycle. The amplitude of the diurnal variations was higher in diabetic rats than in normal rats. V _max values for tyrosine of TAT remarkably increased during the feeding time in both normal and diabetic rats. In both groups K _m values for tyrosine inclined to decrease during the feeding time. In particular, diabetic rats showed a slight but statistically significant decrease in the K _m values of TAT during the feeding time. The overall evidence demonstrated that diurnal variations of the TAT activity were amplified in streptozotocin-induced diabetic rats and therefore suggests that insulin is not involved in developing the rhythmic rise in the TAT activity which results from food intake.