The synthesis of catecholamines and serotonin in the brains of castrated male rats was analyzed at either various times after castration or at various ages. It was found that (a) castration of rats at 50 days or later causes an increase in brain monoamine synthesis, and (b) this phenomenon was not observed until 20 days after castration. The increase in brain monoamine synthesis following castration was counteracted by treatment with testosterone, thus relating the biochemical consequences of castration with changed hormonal conditions of the animal. It is suggested that testosterone exerts an inhibitory influence on monoamine synthesis.