The rate of urinary excretion of lithium was studied in the morning (light) and in the evening (dark) in rats given lithium in the food or by intraperitoneal injection. Lithium clearance was highest during the night. The diurnal variation of lithium clearance was proportional to the diurnal rhythm of glomerular filtration rate. The role of glomerular filtration rate in the diurnal variation of lithium clearance is discussed.