Exposure of male CSF rats to a signalled unpredictable 60-day stress regimen induced a significant elevation in circulating triiodothyronine (T3) concentration above the control for the first 20 days of stress before the rate of secretion returned to normal. On the other hand, circulating thyroxine (T4) concentration fell significantly below the control value for the first 5 days of stress before the normal rate of secretion was reached by 10 days. A histological study of the thyroid gland revealed no change in activity among any of the control and stressed groups. At the same time this regimen also induced an initial extreme corticosterone elevation which was maintained for the first 5 days of stress and thereafter gradually fell to re-establish a new level by 40 days with no further change up to 60 days of stress. The possible physiological significance of the marked elevation in T3/T4 ratio as a result of stress is discussed, in particular with relation to higher glucocorticoid secretion.