Male carps (Cyprinus carpio) of about 1.5 kg were used in all experiments performed in December and February. Blood was taken by heart-puncture within 1 min or less every 4 hr from different fishes during a 24-hr period to establish circadian rhythmicities of all parameters. The water temperature was 11-12 degrees. In December circadian variations could be calculated for cortisol, Na+ and plasma osmolality in partly fed carps and for cortisol, Ca2+, glucose, and plasma osmolality in food deprived animals. No difference in cortisol level, amplitude, or acrophase was present between both experiments but plasma Na+ and osmolality was more elevated when feeding. Also the acrophase of the osmolality rhythm differed between both groups. In February carps, cortisol levels were still comparable to December ones, but a 4 hr shift in acrophase had occurred (from +/- 02 hr to 06 hr). Levels of all other parameters also were comparable to December, except for glucose with only 1/5th of the December values. There was also a shift in acrophase present from the morning to around midnight. The level of cortisol in February carps acclimated to 18 degrees for 1 week was twice the one found in the 11 degrees group. At the same time a significant increase in amplitude and shift in acrophase to 22 hr was seen. Other parameters, except for glucose of which the level was significantly lower than in the 11 degrees group, remained unchanged. Also no correlation between the individual 24-hr data of all parameters could be found. It is therefore concluded that cortisol is not responsible for the observed circadian rhythmicities of Na+, Ca2+, glucose, or plasma osmolality.