Purpose. To evaluate the influence of time of day on the circulating concentrations of 14 frequently used clinical biochemical parameters in the Bispebjerg study of diurnal variations. Materials and methods. Venous blood samples were obtained under controlled environmental, activities and food conditions from 24 healthy young men every third hour through 24 hours, nine time points in total. At each time point, the parameters' concentrations were measured. The data were analyzed by rhythmometric statistical methods and in addition the biological variations were calculated. Results. Significant oscillation of melatonin with an amplitude (amp) of 19.84 pg/ml and a peak at 03:34 h confirmed the normal 24-hour rhythms of the participants. Potassium (p