For decades the most widely described temporal variations were those of blood pressure and urinary excretion, first reported respectively by Zadek in 1881 and Vogel in 1854. Exhaustive reviews of these topics were recently reported by Lemmer'and Cambar et al.* Although variability in blood pressure in physiologic and pathologic conditions is now obvious, the different mechanisms involved in these temporal changes need to be discussed.

Neural, respiratory, cardiac, vascular, renal, endocrine, or behavioral mechanisms all have a regulatory role in blood pressure rhythmicity. Because of the anatomic arrangement of capillaries, the kidneys receive a large proportion of the cardiac output. As a consequence, filtration at the glomeruli is about 25% of the plasma. Therefore, the contribution of the kidneys in the regulation of plasma volume as well as in fluid and electrolyte balance is of great importance. The present review deals with recent knowledge of the renovascular chronostructures, with specific emphasis on the glomerular ones. Circadian changes in renal function, such as the glomerular filtration rate (GFR) and renal plasma flow (RPF), have largely been documented in man as well as in rodents.* After a brief survey of glomerular vessel structure, the relative role of the different parameters contributing to the regulation of GFR and RPF are discussed. We then review, from a theoretical and practical point of view, the different techniques used in patients and rats to precisely assess renal function.