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The effects of certain exogenous hormones on the brood patch of ovariectomized canaries during the non-breeding season are described.

Defeathering and vascularity of the brood patch were produced by oestrogen alone and by combinations of oestrogen and progesterone or prolactin, but not by progesterone or prolactin alone. The development of vascularity varied with the dose level of oestrogen: a dose of 0·2 mg. oestrogen produced stage 3 vascularity, which normally occurs at the time of egg-laying. There was a significant fall in the tactile sensitivity threshold of the brood patch in females treated with oestrogen and progesterone, and in one group treated with progesterone alone. All other groups showed no significant changes.

Oestrogen alone and in combination with progesterone and prolactin caused increase in oviduct weight.

These results confirm the view derived from experiments with intact canaries that oestrogen produces vascularity, but that combinations of oestrogen and prolactin or possibly progesterone are necessary for defeathering. In addition these experiments indicate that oestrogen and progesterone are necessary for a change in sensitivity threshold of the brood patch.