The relationship between fractional sodium excretion (FENa) and fractional lithium excretion (FELi) was determined in Munich-Wistar rats with intact capsules (control, n = 16), and in rats with acute bilateral renal decapsulation (n = 16) during hydropenia and acute saline volume expansion. In response to volume expansion, the glomerular filtration rate increased significantly in both decapsulated and intact groups, but was similar in the two groups of rats at the same period. The FENa and FELi increased significantly from 0.49 +/- 0.10 and 20.09 +/- 1.76% to 1.71 +/- 0.20 and 34.14 +/- 2.82% in control rats with volume expansion. In decapsulated rats, FENa and FELi were 0.17 +/- 0.03 and 11.64 +/- 1.39% during control and increased to 1.04 +/- 0.21 and 26.23 +/- 1.17% during volume expansion. The FELi and FENa were significantly greater in control rats compared with decapsulated rats during both control and volume expansion periods. The lower FELi in bilateral renal decapsulation suggests reduced delivery of sodium from the proximal tubule.