Oestrogen pellets implanted in castrated or spayed rats cause both the males and females to take a considerable amount of voluntary exercise. Testosterone pellets cause implanted spayed females to take considerable exercise but their effect on the castrated male is much less. If the male has previously been under the influence of an oestrogen pellet, the amount of activity that results when a testosterone pellet is implanted is increased. The suggestion is made that once the responsible brain centre has been exposed to oestrogen it becomes sensitive to the action of testosterone.
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