A single injection into laying hens of 60 mg metyrapone 28 h after the final ovulation of a sequence induced increases in the plasma concentrations of LH and progesterone, followed by premature ovulation. Injection of metyrapone 8 h after ovulation, however, did not affect plasma concentrations of either LH or progesterone. 2. Injection of laying hens with 60 mg metyrapone on 5 successive days reduced the effectiveness of exogenous ACTH in increasing the plasma concentration of corticosterone and abolished the system of "open" and "closed periods" for pre-ovulatory LH release. Thus, pre-ovulatory LH surges and ovipositions occurred throughout the 24-h day instead of being restricted to an 8 to 10-h period of the day. 3. These observations suggest that changes in environmental stimuli such as light act via the adrenal gland in regulating the timing of the "open period" for the pre-ovulatory release of LH in the hen.
