Circadian studies of plasma cortisol, thyroid hormone, protein, glucose and ion concentration, liver glycogen concentration and liver and spleen weight in rainbow trout, Salmo gairdneri Richardson.

1. Although there are many reports in the literature of circadian rhythms in plasma hormone and metabolite levels, the data are highly variable between research groups. The present study attempts to re-examine whether circadian rhythm in plasma cortisol, thyroid hormone, ions, glucose and protein levels and liver glycogen levels are evident in rainbow trout, Salmo gairdneri, and determine whether there is a significant correlation between any of the measured variables. 2. Significant fluctuations throughout the day were found in all measured variables; although these fluctuations appear to be a normal component in the homeostatic function of rainbow trout, their timing was neither consistent nor predictable. 3. "Circadian-like" patterns were observed in levels of plasma cortisol, glucose, Mg2+ and K+ concentrations and liver glycogen concentration. 4. Seasonal variations in these circadian-like rhythms were found in liver glycogen and plasma cortisol, Mg2+ and K+ concentrations. 5. Plasma cortisol and glucose concentrations were affected by time of feeding. 6. There were significant correlations between plasma thyroid hormone and plasma protein levels, but no other significant correlation between any of the measured variables was found.