Since part of the natriuresis of the extracellular fluid volume expansion (ECVE) is due to renal vasodilatation, it is possible that the blunted natriuresis during ECVE in dogs with chronic ligation of common bile duct (CBD) is due to gailure to renal arteries to dilate. Studies were carried out to investigate this question in normal dogs, sham-operated animals, and dogs with acute and chronic legation of CBD. ECVE with 0.45 percent saline caused a significant rise in renal plasma flow (RPF) and urinary sodium excretion (UNaV) in all animals except those with chronic ligation of CBD. In the latter dogs, the increment in UNaV was significantly less than that observed in the other groups of animals, and RPF did not increase significantly following saline infusion in dogs with CBD ligation. The intrarenal administration of acetylcholine produced a significant and comparable increase in RPF in all animals, including those with chronic ligation of CBS, but the rise in UNaV in the latter was significantly less than that in the other groups of dogs. The data indicated that (1) ECVE is not associated with renal vasodilatation in dogs with chronic ligation of CBD, (2) this abnormality is not due to a defect of the renal arteries to respond to vasodilators, and (3) the lack of renal vasodilatation may be partly responsible for the blunted natriuresis, but other sodium-retaining forces may also be operative.