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Varying doses of L-phenylalanine were administered intraperitoneally to rats and motor activity determined by means of photocells in an activity cage. Control rats, and rats whose brain catecholamines were depleted by reserpine, α-methyl-p-tyrosine, or intraventricular 6-hydroxydopamine showed a decrease or no change in motor activity following phenylalanine injections. Injections of the decarboxylase inhibitor, Ro 4–4602, failed to alter the effect of phenylalanine. After injections of [3 H]phenylalanine, [3 H] β-phenethylamine accounted for less than 5 % of brain tritium and radioactivity associated with this amine declined rapidly with time. The proportion of brain radioactivity due to β-phenethylamine was increased by the monoamine oxidase inhibitor, pargyline, and was not affected by the peripheral decarboxylase inhibitor Ro 4–4602. When Ro 4–4602 and pargyline were given together, the increase in radioactive phenethylamine produced by pargyline was prevented, suggesting that a significant proportion of brain phenethylamine may be synthesized in the periphery and can enter brain if monoamine oxidase is inhibited.