Rainbow trout(Salmo gairdneri)were adapted to long (LD = 12:12) or short (LD = 8:16) daily photoperiods and pituitary, plasma and tissues were collected from fish sampled over a 24 h period to examine possible diurnal variation in pituitary somatotrop activity, plasma free fatty acid (FFA) concentration and liver, muscle and carcass lipid content. Trout acclimated to both long and short daily photoperiods showed a similar significant daily cycle of somatotrop activity with peak activity occurring during the second half of the dark phase. Plasma FFA levels in trout on a long photoperiod similarly showed a significant diurnal variation and there was a significantdirectcorrelation between somatotrop activity and plasma FFA concentration, whereas in trout acclimated to a short daily photoperiod plasma FFA levels did not vary diurnally and there was a significantinversecorrelation between somatotrop activity and plasma FFA levels. There were no consistent diurnal changes in liver, muscle or carcas lipid content with the possible exception of an increase in muscle lipid content in the second half of the dark period in trout acclimated to the short daily photoperiod.