Ninety-six pigs, half females and half castrated males from 12 litters, were housed in 24 groups of four litter mates. From an age of 115 days half of the groups were subjected to chronic stress for 33 days consisting of a schedule of unpredictable, inescapable electroshocks, and half served as controls. Behavior and performance were measured on all animals in the group, hormone data on one female in each group, and data on ulceration on the castrates. Behaviorally, the pigs did not habituate to 31 days of stress treatment. One to 2 days of stress treatment produced a behavioral activation that after 9 to 10 days was restricted to the time period of potential stress treatment. After 30 to 31 days it was replaced by passive behavior. In the beginning as well as after 30 to 31 days of intermittent stress, time spent sitting was increased. In addition, 6 days of intermittent stress reduced the baseline mean of plasma ACTH. After 33 days of stress the baseline mean of plasma ACTH was normalized, but the time course of diurnal secretion of ACTH was shifted. No effects of the chronic intermittent stress on basic levels of plasma cortisol, performance, or gastric ulceration were evident. In conclusion, the effect of intermittent stress depends on the number of days of intermittent stress treatment and it does not inevitably include changes in the basic levels of cortisol. Thus, assessment of stress must be based on a wide range of variables describing the process.