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The possibility that adrenaline may be synthesized locally in the eye was investigated in the rabbit and vervet monkey. L-tyrosine-3,5-3H, L-tyrosine (side-chain-2,3-3H) or L-3,4-dihydroxyphenylalanine (3H(G)) was injected into the anterior chamber or the vitreous. The animals were killed after 1/2 h up to 7 days in the tyrosine experiments and after 1 up to 24 h in the DOPA experiments. The synthesis of 3H-adrenaline, 3H-noradrenaline and 3H-dopamine was measured with a modified version of a double isotope technique for determination of catecholamines. No 3H-adrenaline was found. An attempt to stimulate the phenylethanolamine-N-methyltransferase activity with a glucocorticoid and to inhibit the metabolism of catecholamines with a monoaminoxidase inhibitor also failed to demonstrate 3H-adrenaline synthesis. 3H-noradrenaline was found in all experiments when 3H-DOPA had been given. The amounts were about 10 times higher after injection into the anterior chamber than after injection into the vitreous. 3H-dopamine was found in all except a few experiments after 3H-DOPA injection. The highest amounts were found after injection into the vitreous. In experiments with 3H-tyrosine we were unable to demonstrate any catecholamine synthesis. This prompts the question of whether tyrosine-hydroxylase may be absent in the eyes of certain animal species.