To elucidate possible mechanisms behind the endocrine control of parr-smolt transformation, the daily plasma profiles in thyroid hormones (TH; free thyroxine (FT(4)), total thyroxine (TT(4)), and total 3,5,3'-triiodothyronine (TT(3))), growth hormone (GH) and cortisol were studied in Atlantic salmon parr and smolts under simulated-natural winter (8 L:16D) and spring (16.5 L:7.5D) photoperiods, respectively. Overall, TT(4), TT(3) and GH levels were higher in smolts than in parr, whereas FT(4) levels fluctuated within the same range in parr and smolts. Significant diurnal changes in plasma TH were present in parr. Both FT(4) and TT(4) levels increased during the photophase and decreased during the scotophase, while TT(3) levels followed an inverse pattern. Growth hormone showed no significant changes in parr. Changes in FT(4), TT(4), GH, and cortisol, but not TT(3), levels, were observed in smolts with peak levels during both the photophase and scotophase for FT(4), TT(4) and GH. Plasma cortisol was not assayed in parr but in smolts the peaks were associated with dusk and dawn. In addition to the general increases in TH, GH and cortisol, the distinct endocrine differences in nighttime levels between parr in the winter and smolts in the spring suggest different interactions between TH, GH, cortisol and melatonin at these different time points. These spring scotophase endocrine profiles may represent synergistic hormone interactions that promote smolt development, similar to the synergistic endocrine interactions shown to accelerate anuran metamorphosis. The variations in these diurnal rhythms between parr and smolts may represent part of the endocrine mechanism for the translation of seasonal information during salmon smoltification.