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Groups of laying mature domestic fowl were injected i.m. with varying doses of either oestradiol benzoate, testosterone propionate or progesterone and were killed at random intervals throughout the day without reference to any specific point in the ovulatory cycle. Luteinizing hormone (LH) was assayed in the anterior pituitaries of each group. It was shown that oestradiol in doses calculated to be equal to or above the physiological level, increased pituitary LH without necessarily affecting the laying cycle. Testosterone had no significant effect on pituitary LH, while progesterone significantly increased pituitary LH at doses which had no apparent effect upon ovulation. Doses effective in the laying hen had little or no effect on the pituitary content of LH in immature birds. The results in the laying hen are in harmony with the concept previously proposed, by which changes in the levels of circulating plasma oestrogens may regulate the ovulatory pattern of the fowl by inhibiting release of pituitary LH.