Ureagenesis from ammonia, alanine, and glutamine in the liver after partial hepatectomy (PH) was determined by using the liver-perfusion system. The maximum rate of ureagenesis from ammonium chloride (10 mmol/L) in hepatectomized (HX) rats at 24 hours after surgery was obtained in the presence of ornithine, lactate, and pyruvate, and it was almost identical to that in sham-operated (SO) rats. The rate of urea production from glutamine (1 mmol/L or 10 mmol/L) in HX rats was significantly lower than that of SO rats with a concomitant decrease in hepatic glutaminase activities. However, the rate of urea synthesis from glutamine (1 mmol/L) in the presence of added ammonia (0.5 mmol/L) was accelerated approximately 10-fold, and the significant difference in the rate of urea formation between HX and SO rats was abolished. This result indicates that there is enough glutaminase to generate ammonia from glutamine in the liver of HX rats. The rate of urea production from alanine (1 mmol/L or 10 mmol/L) in HX rats was significantly decreased at 24 hours following surgery, while that of SO rats was increased. The decreased formation of urea from alanine was not seen at 72 and 120 hours after the operation. These results suggest that during the proliferation phase of liver regeneration, a reduction of ureagenesis from alanine facilitates the remnant liver to make nonessential amino acids such as aspartate. This metabolic alteration might be related to the proliferation of liver cells.