Reports on diurnal variation in plasma corticosteroids in man, monkey, dog, rat, mouse and other species have been reviewed by Yates, Russell & Maran (1971) and by Yates & Urquhart (1962). We wish to report evidence of diurnal variation in the peripheral plasma cortisol levels of sheep, demonstrable after they had been allowed adequate time to adapt to the environment in which they were tested.
Three anoestrous New Zealand Romney ewes were placed indoors in crates with minimal restraints. They were placed immediately adjacent to each other and were free to sit or stand. They were given hay, lucerne pellets and water ad libitum. No attempt was made to regulate feeding and an excess of feed was ensured at all times. Each animal had an indwelling jugular cannula, inserted at least 3 h before the first sampling, which remained in situ throughout the experiment. On each occasion 10 ml blood