Handling and confinement caused a steady increase in the plasma ACTH level in both coho salmon and rainbow trout. Within 2 min plasma ACTH levels had increased significantly, and by 30 min they were 5- to 8-fold higher than the basal ACTH level in unstressed fish. This type of stress also caused a pronounced elevation in plasma cortisol, which lagged behind the ACTH increase, although the degree of change was greater, the level rising between 20- and 50-fold. The plasma alpha-MSH level was unaffected by handling and confinement stress. A second series of experiments assessed the effects of a more severe stress, which consisted of 5 min out of water, during which the fish were restrained, followed by 25-min confinement in a small volume of water. This caused a very rapid, pronounced increase in the plasma ACTH level of sterile rainbow trout, the level reaching a peak at 5 min, and remaining elevated for the next 25 min. Plasma cortisol levels, which were low at the beginning of the experiment, remained so for the first 5 min, and rose thereafter. This type of stress also caused a rapid and pronounced elevation of the plasma alpha-MSH level. It rose in a very similar way, and at the same time, as the plasma ACTH level, but instead of remaining elevated it fell during the 25 min of confinement which followed the 5 min of restraint, to finish one-third of the peak value reached after 5 min.