Recently it was suggested that the onset of sodium retention in experimental cirrhosis in rats is related to a critical threshold of hepatic function, as assessed by the aminopyrine breath test. The aim of this study was to evaluate whether sodium retention occurred after two-thirds hepatectomy in rats and to investigate the relationship between sodium retention and changes in hepatic function associated with liver regeneration in this model. Sodium balance, creatinine clearance, serum sodium and the aminopyrine rate constant of elimination were evaluated daily for 4 days after surgery in partially hepatectomized (n = 6) and shamoperated rats (n = 6). All rats in the partial hepatectomy group exhibited sodium retention (sodium balance >0.7 mmol/day) 24 hr after surgery. This was associated with a 62% reduction of the aminopyrine rate constant of elimination. Spontaneous natriuresis, which occurred between 2 and 4 days after surgery, was associated with an increase in the aminopyrine rate constant of elimination (from 0.73 ± 0.02 × 10−2 min−1 on the last day of sodium retention to 0.95 ± 0.04 × 10−2 min−1 on the first day of natriuresis [p < 0.05]). In contrast, no change in creatinine clearance occurred over the same period. A negative curvilinear association was found between sodium balance and the aminopyrine rate constant of elimination in all animals (r = −0.72, p < 0.001). These observations indicate that natriuresis is related to the recovery of liver function, not to changes in creatinine clearance. In conclusion, the concept of a critical threshold of liver function below which sodium retention occurs has been substantiated in this model of hepatic dysfunction.