The influence of autumnal progressive and direct seawater transfers on ionic parameters, plasma growth hormone (GH) and thyroid hormones (TH) and also on the non-specific immune traits phagocytic activity, lysozyme and non-specific cytotoxicity were examined in 45-55 g brown trout (Salmo trutta). In both experiments, the seawater transfer induced the same pattern of endogeneous modifications but they were more pronounced and more lasting after the direct seawater transfer than after the progressive one. In seawater-transferred trout, there was a significant transitory increase of the plasma osmolarity, chloride concentration, GH levels and a transient decrease of the TH. The phagocytic activity of the pronephric leucocytes and the lysozyme concentrations were significantly higher in seawater-transferred trout than in controls. Nevertheless, the non-specific cytotoxicity should not be modified after the seawater exposure. Moreover significant positive correlations were observed between plasma GH and chemiluminescence or lysozyme increases. These data support the hypothesis that GH is involved in the salmonids' non-specific immune potential, especially by stimulating the macrophage functions.