Atlantic salmon parr and smolts reared under a natural temperature and photoperiod regime were subjected to an acute handling and confinement stress in early May. Smolts had a mean plasma cortisol concentration of 10 ng/ml before stress and 242 ng/ml 3 h after initiation of stress which returned to pre-stress levels within 8 h. Parr had a plasma cortisol concentration of 4 ng/ml prior to stress which increased to 11 ng/ml 3 h after initiation of stress and returned to pre-stress levels within 8 h. Plasma glucose was significantly higher in parr and smolts 3 h after initiation of stress; in parr, plasma glucose returned to pre-stress levels within 8 h, but not until 48 h in smolts. Plasma chloride concentration in smolts decreased from 139 to 124 mM 3 h after initiation of stress but returned to pre-stress levels within 24 h; plasma chloride in parr was not altered by stress. Plasma thyroxine of parr and smolts peaked at 3 h after initiation of stress and returned to pre-stress levels within 8 h, but smolts had 72% higher levels at 3 h. Pre-smolts (February) and smolts (May) reared under constant temperature (8–10°C) were also subjected to a handling and confinement stress. Although peak levels of plasma cortisol 3 h after initiation of stress were twice as high in smolts, other physiological and endocrine responses were not substantially different between pre-smolts and smolts. The results demonstrate that Atlantic salmon smolts are more responsive to stress than parr and that developmental differences are more important than seasonal changes.