Diurnal variation of cortisol in the peripheral plasma of sheep has been demonstrated after the animals had been allowed to adapt to the environment in which they were tested. This variation was not seen in sheep tested after only 7 days in their new environment (McNatty, Cashmore & Young, 1972). Although adapted sheep have higher plasma levels of cortisol in the morning daylight hours than during the remainder of the 24-h day (McNatty et al. 1972), it is not known whether this diurnal variation is re-established while sheep are habituating to their new environment. We have limited evidence to suggest that sheep require 14–28 days to become adapted to a new environment (D. C. Thurley & K. P. McNatty, unpublished observation). We wish to describe the diurnal changes in the peripheral plasma cortisol levels of sheep while they are adapting to a new environment and exposed to diffuse light. Nine