This study established the existence of a parr-smolt transformation in the anadromous form of brown trout but no such change in the resident form when reared under ambient photoperiod and temperature. Smolting occurred in March-April in the larger trout of the anadromous form and in April-May in salmon. Silvering, a peak of gill Na+,K+-ATPase activity and a simultaneous decrease in plasma prolactin were observed in both salmon and the anadromous form of trout. Bimodality in length frequency and a surge in growth hormone were observed only in Atlantic salmon. No physiological or morphological changes were observed in smaller (< 13 cm) trout of the anadromous form or in the resident form. Hypo-osmoregulatory ability was greatest at the time of peak gill Na+,K+-ATPase activity and increased in spring in all trout, irrespective of smolt status. It is concluded that smolting is not as well developed in brown trout as in Atlantic salmon, and is not necessary for seawater adaptation of brown trout. It is also dependent on both stock origin and fish size.