Effects of feed restriction and stress on smolting of hatchery reared Salmo trutta were studied under experimental conditions. The handling periods in these two experiments took place over the summer and autumn, which is believed to be the sensitive period for the physiological decision of smolting. Possible changes in physiology and behaviour were observed during the following spring and summer. Smolting was not significantly affected by these handling procedures. However, drastic changes in movement behaviour of individually tagged trout during May and June were observed. Before mid-May downward migration suddenly increased, peaked at the end of May and beginning of June, and abruptly ceased in the mid-June when the temperature rose above 10°C. The behavioural pattern of both sea trout and brown trout was similar. The speed of the migration during the most intense period was almost equal to the speed of water, and most movement took place during daylight irrespective of the water temperature. This result was supported by the observation that both 1+ and 2+ brown trout feed most avidly during the daylight hours. During the smolting period also typical changes in gill Na⁺, K⁺-ATP-ase activities were observed. Highest gill Na⁺, K⁺-ATP-ase activities were measured at the end of May, when those were significantly higher in brown trout than in sea trout.