Epidermal growth factor (EGF) was measured in milk from four groups of rats: untreated diabetic, insulin-treated diabetic, insulin-treated normal and control rats. In the untreated diabetic group the volume of milk, and the concentration of EGF and the total output of EGF were significantly decreased when compared to the control group. In contrast, the total protein concentration in milk from the untreated diabetic rats was similar to the concentration in milk from the control rats. Insulin-treatment of diabetic rats almost completely reversed the decrease in the milk volume and in the concentration of EGF, and thus normalized the total output of EGF. The insulin-treated normal rats which remained euglycemic had a significantly increased concentration of EGF and of total protein without any difference in the volume of milk when compared to the controls. The results indicate that the secretion of EGF from the mammary glands is dependent on insulin and that the decrement in milk-EGF from diabetic rats is selective when compared to the content of protein in milk.