Atlantic salmon parr and pre-smelts, reared under natural conditions in fresh water (FW) were implanted with an intraperitoneal cholesterol pellet containing either ovine growth hormone (oGH, 7 mu g/g for pre-smolts and 18 mu g/g for parr), or no hormone for control fish, 12 or 3 months respectively before natural completion of their parr-smolt transformation. Twelve days after treatment, fish were directly transferred into full salinity sea water (SW, 35 ppt), In FW, both parr and pre-smolts had normal daily rhythms of oxygen consumption (MO(2)), Treatment with oGH caused a greater increase in mean MO(2) in pre-smolts (+30%) than in parr (+16%), which was illustrated by an upward shift of the curves representing daily variation of oxygen demand, In addition, gill (Na+-K+)ATPase activity increased by +50% in parr and +184% in pre-smolts treated with oGH. In parr, the increased salinity caused a reduction in MO(2). This effect, together with the disappearance of the rhythm of oxygen consumption in control parr, could be regarded as the consequence of the profound physiological disturbance caused by the abrupt transfer to SW suggesting that these fish were not totally adapted to SW. The attenuated decrease in MO(2) in treated SW-transferred parr (-27% compared with -50% in controls) and the increase in MO(2), in treated SW-transferred pre-smolts (+13% compared with +5% controls), underline the positive effect of oGH on SW adaptability in parr and especially pre-smolts, This interpretation is supported by the improved ionic regulation in all treated fish. Moreover, changes in the plasma ion concentrations in parr may indicate a metabolic acidosis which, at least partly, may explain the marked fall of MO(2) observed after SW transfer.