Eight growing crossbred beef heifers, surgically fitted with indwelling abomasal cannulas, were used in a 4 x 4 Latin square design and examined for the effects of continuous abomasal infusion of arginine and(or) ammonium acetate on liver arginase activity and urea and ammonia metabolism. Heifers were fed complete diets containing 1.2% urea as the only source of supplemental N. Infusion treatments were: (1) control (C): 2 liters water/24 h; (2) arginine (Arg): 24.00 g arginine in 2 liters water/24 h; (3) ammonium acetate (NH4Ac): 116.34 g ammonium acetate in 2 liters water/24 h; (4) arginine plus ammonium acetate (Arg + NH4Ac): 24.00 g arginine . HCl plus 116.34 g ammonium acetate in 2 liters water/24 h. Infusions solutions contained 0, 1, 1 and 2% crude protein equivalent, respectively, above the level of crude protein provided by the diet. NH4Ac and Arg + NH4Ac increased (P less than .05) apparent digestibility of N over that observed with treatment C. Arg tended to increase N retention over that observed with C, while both NH4Ac and Arg + NH4Ac resulted in lower (P less than .05) N retention than did C and Arg. Mean liver arginase activities (mumol . mg protein-1 . h-1) were 166.9, 137.5, 226.3 and 194.5 for heifers infused with C, Arg, NH4Ac and Arg + NH4Ac, respectively. NH4Ac and Arg + NH4Ac resulted in higher (P less than .05) arginase activity than did C or Arg. Arg infusion increased plasma urea N (PUN) over that observed with C, while PUN was higher (P less than .05) with NH4Ac or Arg + NH4Ac than with C or Arg. NH4Ac infusion increased (P less than .05) plasma ammonia-N (PAN) over that observed with Arg or Arg + NH4Ac, while PAN levels were the same with C, Arg and Arg + NH4Ac. Both Arg and NH4Ac infusion resulted in higher (P less than .05) rumen ammonia levels than C, while heifers given Arg + NH4Ac had rumen ammonia levels similar to those of controls. Mean daily urea excretion was 13.16, 16.02, 28.92 and 32.44 g/24 h for heifers infused with C, Arg, NH4Ac and Arg + NH4Ac, respectively. The major difference in excretion of urea was attributable to NH4Ac and not Arg. Daily urea excretion decreased between 1530 and 2330 h and increased between 2330 and 0730 h. Arginine infusion appeared to have a protective effect against ammonia infusion because PUN increased and PAN decreased in heifers infused with arginine. The reduction (P less than .05) in plasma amino acids thought to be limiting for protein synthesis in ruminants in conjunction with increased (P less than .05) N retention and the trend toward decreased arginase activity observed in heifers infused with Arg suggests that the effects of arginine on ammonia metabolism are not wholly mediated by liver urea synthesis.