Variations in plasma prolactin, thyroid hormones, gonadal steroids and growth hormone in turkeys during the induction of egg laying and moult by different photoperiods.

1. Concentrations of prolactin, growth hormone, testosterone, progesterone, thyroxine and triiodothyronine were measured in the blood plasma of female turkeys during successive periods of egg laying, a decline in lay, a moult induced by a short photoperiod (6 light: 18 dark) and a resumption of egg laying induced by a long photoperiod (16L:8D). 2. Concentrations of prolactin, growth hormone, testosterone and progesterone were higher in laying birds than in birds which were moulting or not laying. 3. The concentration of testosterone, but not of the other hormones studied, increased significantly during the period of profuse moult. 4. Concentrations of the thyroid hormones did not change with the varying physiological condition of the birds. However, the concentration of thyroxine was depressed by the long photoperiod.