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Abstract
1. Circulating immunoreactive luteinising hormone (LH) and follicle stimulating hormone (FSH) concentrations have been measured during photoperiodically‐induced changes in the reproductive state of turkeys.
2. In a period of sexual quiescence on short photoperiods (6L : 18D) LH concentrations were higher during the hours of darkness in both sexes.
3. Transfer to long photoperiods (16L : 8D) stimulated a rapid increase in LH and FSH concentrations. This was maintained for between 2 to 3 months when the concentrations of both gonadotrophins decreased as the birds became photorefractory.
4. Return to short photoperiods had no immediate effect on the concentrations of LH and FSH in females. The concentration of LH was increased during the 3rd week of short photoperiods when the hens were moulting.
5. LH concentrations fluctuated during the ovulatory cycle and were highest about 6 h before ovulation.