Atlantic salmon reared under natural conditions in a fresh water (FW) hatchery at Le Conquet (Brittany, 48-degrees-N) were treated with different hormones in order to trigger or advance the parr-smolt transformation and to improve seawater (SW) adaptability. In the first experiment, juveniles were implanted with ovine growth hormone (oGH) pellets (7 mug/g body weight for pre-smolts, 25 mug/g for parr) and compared with sham-operated and control salmon at different times of the year. Different results were observed following direct exposure to full salinity (35 parts per thousand) SW depending on the date of treatment. Prior to smoltification (November, December and February), pre-smolts treated with oGH had higher gill Na+,K+-ATPase activity in FW, lower plasma osmolarity after SW transfer, and higher growth rate in SW compared with sham and control fish. However, for pre-smolts and smolts treated during April, May or June, there was no difference between treated and untreated fish. At any time of year, oGH-implanted parr survived SW transfer better than sham and control fish. Using a recombinant trout growth hormone (rtGH) at lower doses (0.28 and 1.4 mug/g), a high percentage survival was observed after direct SW transfer for both pre-smolts (100% after 30 days in SW compared with 86% in sham) and parr (65% compared with 0%). rtGH is at least 10 times more efficient than oGH for increasing gill Na+,K+-ATPase and salinity tolerance. Treatments with cortisol (8 mu/g in silastic pellets), 3,5,3'-triiodo-L-thyronine (T3, 20 mg/kg of food during 6 weeks), ovine prolactin (oPRL, 7 mug/g) and oGH (7 mug/g) were used on pre-smolts in October (8 months old). After direct SW exposure, only oGH-treated fish (alone or in combination with other hormones) were able to adapt, survive and grow. Both oPRL and oGH treatment increased gill Na+,K+-ATPase activity in fish in FW; however, there was no improvement of SW adaptability in oPRL-implanted salmon.