The diurnal patterns of plasma growth hormone (GH), thyroid hormone and cortisol concentrations in rainbow trout Oncorhynchus mykiss held under three photoperiod (Lβ:βD) regimes (6β:β18, 12β:β12 and 18β:β6), and fed either daily (DF) or on alternate days (ADF) with 2Β·0% body mass per day of a commercial trout diet were determined. The ADF groups had reduced total mass gain and specific growth rates compared with DF fish, but photoperiod had no affect on growth for either of the feeding regime groups. In the ADF groups, the mean 24βh plasma thyroxine (T~4~) and triiodothyronine (T~3~) concentrations were significantly lower, both on days of feeding and days of fasting, than in DF fish held under all three photoperiod regimes, but for GH, only the 18Lβ:β6D DF group was higher than the comparable ADF groups. There were no significant differences in mean 24βh plasma cortisol concentrations of DF and ADF groups. Diurnal patterns of plasma GH, cortisol, T~4~ and T~3~ were found in DF fish held under all three photoperiod regimes. Increases in plasma cortisol changes were associated with the onset of the light phase; elevations in plasma GH and T~4~ concentrations were more closely associated with clock time, regardless of photoperiod; increases in plasma T~3~ concentrations were strongly associated with time of feeding. In ADF groups, these diurnal changes in plasma GH, T~4~ and T~3~ concentrations were suppressed for both the fed and fast days, and plasma cortisol concentrations were suppressed on the fasting day. The observations are discussed in terms of the proposed anabolic, catabolic and growth regulating roles of these hormones in different growth and metabolic modifying situations in teleosts.