The present study evidences blood and urine urea, creatinine and total proteins circadian variations in 50 male rats. Venous blood is sampled at behind socket sinus once a week at different hours (8, 11, 14, 17, 20, 23, 2 and 5) and urines are collected during 4 consecutive six hours periods in animals living in metabolism cages (8-14, 14-20, 20-2, 2-8). Blood three nitrogen substances circadian variations bring out the decrease at 17 h and increase at 23 or 2 h. Urinary excretion variations curves shows, in all cases, an increase more than 40% during the nightly periods. The influence of feeding rhythms on the blood and urine three derivates circadian rhythms is discussed. Moreover, night diuresis increase and urea and creatinine urinary remarkable constancy suggest water, solutes and macrosolutes transglomerular pathway nightly increase existence. Urea (+ 40%) and creatinine (+ 30%) clearance nightly significative increase confirms glomerular filtration circadian variations and its nightly increase.