Wistar strain albino male rats (100g-150g) were injected intramuscularly with insulin (0.1 IU per 100g body weight). Quantitative analysis of phenylalanine and tyrosine in plasma and the brain cortex, and of catecholamines and tyramine in plasma and whole brain was performed. The activity of tyrosine hydroxylase in brain striata and the adrenal glands, and the activities of tyrosine decarboxylase and aromatic amino acid decarboxylase in both the whole brain (except cerebellum) and the liver were also studied. The quantitative analysis of plasma phenylalanine wass performed using the method of McCaman and Robins. Tyrosine was analysed by the method of Waalkes and Udenfriend. Tyrosine hydroxylase activity was measured by the method of Coyle, and aromatic amino acid decarboxylase activity by the method of Christenson. Catecholamines were quantitated by the method of Kawai. All activities (Results 1-4) were determined 60 minutes after the insulin injection, the time by which hypoglycemia had definitely developed. The results were as follows; 1. Blood glucose decreased to 47.3% sixty minutes after the insulin injection. Phenylalanine in plasma and in the brain decreased significantly but tyrosine levels did not show any change. 2. In the blood, dopamine and norepinephrine decreased significantly whereas epinephrine and tyramine increased significantly. In whole brain (except cerebellum), dopamine and norepinephrine decreased significantly, tyramine increased significantly, but epinephrine increased only slightly. 3. Tyrosine hydroxylase activities in striata and in the adrenal glands increased significantly. 4. Tyrosine decarboxylation in diencephalon combined with striata and in the liver was also high. 5. Tyrosine hydroxylase activity in whole brain (except cerebellum) was high 60 minutes to 90 minutes after the insulin injection. Tyrosine hydroxylase activity in the adrenal glands was high 30 minutes to 90 minutes after the insulin injection. 6. Tyrosine decarboxylation in the whole brain (except cerebellum) and in the liver was significantly high 30 minutes to 60 minutes after the insulin injection. When L-DOPA was used as the substrate, the activity of aromatic amino acid decarboxylase in the brain increased 90 minutes after the insulin injection. The activity of this enzyme in the liver, however, was not different from that of in the liver of the control rats. These results suggest that insulin causes an increase in the activity of tyrosine hydroxylase and an increase in the production of catecholamines. It also accelerates the metabolic pathway from tyrosine to tyramine by increasing the activity of tyrosine decarboxylase.