A possible role of nutrition as a synchronizer has been recently emphasized, particularly the effect of controlled diet composition of circadian variations of many functions of the organism. The aim of the study was to determine whether diet composition (low or high fat diet) could be a synchronizer of circadian rhythms of glucose, insulin and triiodothyronine. The effect of diet composition on diurnal changes in glucose tolerance was also tested. After 24 h starvation period the biochemical parameters in the serum were measured every 4 hours i.e. 600, 1000, 1400, 1800, 2200, 200. Glucose tolerance was tested between 500-700, 1100-1300, 1700-1900 and 2300-100. The circadian variations of glucose and insulin levels were observed in animals fed both diets. An increase of glucose level was noted during reduced activity of the animals and the acrophase was recorded at 1659 h (low fat diet) and 1514 h (high fat diet). The acrophase of insulin level was observed at 526 h (low fat diet) and 352 hrs (high fat diet) in the period of activity of the animals. Circadian changes of triiodothyronine level were noted in animals fed the low fat diet only, the acrophase appeared at 1447. Simultaneously, no variations occurred in animals fed the high fat diet. A consequence of the high fat diet was also a disappearance of diurnal variations in glucose tolerance test at 60, 90 and 120 min.