1. The levels of four hormones which might be involved in the control of growth have been measured in the blood of lambs kept in 16 or 8 h daylengths for 4 months in each of two experiments. Under the conditions of these experiments long daylength stimulated growth.
2. Prolactin concentration was significantly increased by long day-length and by ad libitum feeding, compared with short daylength and restricted feeding, respectively.
3. Growth hormone, insulin and thyroxine concentrations were unaffected by daylength. Ad libitum feeding caused lower growth hormone and higher insulin concentrations than restricted feeding, and growth hormone levels fell after the morning feed.
4. Of the hormones measured, prolactin is most worthy of further study as a possible mediator of the effects of long daylength on growth.