Diurnal variations in 23 blood chemical items were investigated in Sprague-Dawley rats of both sexes at 6 weeks of age. Blood samples were obtained every two hours. One to three periodic components detected by the maximum entropy method from the observed time-series and the optimum fitting parameters were calculated by the least squares method. The fitting curves were compared with the observed data for the evaluation of systematic and fluctuating parts of the time domain as circadian variations. The periodic components were detected from all the blood chemical items examined. This suggests that all items have systematic parts as basic rhythmic changes. The optimum fitting curves corresponded to the observed time-series in the following items: glucose, total protein, A/G ratio, alkaline phosphatase, Na, K, Cl and BUN of both sexes and total cholesterol, triglyceride, cholinesterase, CPK, inorganic phosphorus, direct and total bilirubin, creatinine and uric acid in males. Others showed insufficient correspondence to many fluctuating parts. This randomness was more frequently seen in females than in males.