The delivery of tubular fluid from the proximal straight tubule to the thin descending limb of the loop of Henle was measured in nine volunteers by the lithium clearance method and by the water diuresis method. Lithium clearance and urine flow during water diuresis varied in proportion to each other with a high degree of correlation (r = 0.93, p less than 0.001). The proportionality between lithium clearance and urine flow was unaffected by variations in fractional sodium excretion. The results support the assumption that lithium clearance can be used as a measure of the delivery of tubular fluid from the proximal tubule in humans with sodium intakes within the normal range.