Plasma somatolactin (SL) concentrations in rainbow trout were examined under various physiological and environmental conditions. Background adaptation and feeding did not affect plasma SL levels. There was no consistent change in plasma SL levels during fasting for 21 days, although increased plasma growth hormone levels and decreased condition factor, hepatosomatic index and abdominal fat, occurred. Plasma SL concentrations increased during acute stress and also during exhaustive exercise resulting from being chased in shallow water. Elevation of plasma SL was associated with those of plasma cortisol, Ca2+, phosphate, and glucose levels. On the other hand, plasma level of prolactin was not affected in the stress and exercise experiments, although plasma GH and Na+ were raised in the fish 5 min after the onset of the stress. Our results suggest the involvement of SL in calcium and phosphate metabolism, acid-base regulation, or energy mobilization in the stressed or exercised trout.