Dexamethasone suppression of the plasma corticosterone response to stress in the rat.

1. The effect of dexamethasone on the plasma corticosterone response to heat-immobilization stress was investigated in male and female rats.2. The plasma corticosterone response of control rats to the type of stress described was 64 mug% (+/- 10.5 mug% standard deviation) in males, and 128 mug% in females.3. A straight line relation between the dose of dexamethasone and the log of the plasma corticosterone response was demonstrated in male rats. The slope of the dose-log response line was used to indicate the degree of suppression of the plasma corticosterone response to stress.4. The straight line relation did not hold for female rats, suggesting the presence of an additional factor or factors which influenced the plasma corticosterone levels.5. An exponential relation between the duration of dexamethasone administration and the slope of the dose-log response line was demonstrated, maximum suppression occurring after an infinite period of time. After 30 days, 95% maximum suppression occurred when the plasma corticosterone response was measured 8 hr after dexamethasone administration had ceased.