Two experiments were conducted with young, growing pigs to evaluate the effects of protein levels and supplemental lysine on growth, body characteristics and urea cycle enzyme activities. In the first experiment, 8-week-old pigs weighing 17 kg were fed two levels of protein (12 or 20%) until they reached slaughter weight of approximately 95 kg. In the second experiment, three levels of protein (12, 18 or 24%) and one level of additional lysine (9 g/kg of diet added to the 12% protein diet) were fed for the same interval. After slaughter, urea cycle enzyme activities (argininosuccinate synthetase, EC 6.3.4.5; argininosuccinate lyase, EC 4.3.2.1; and arginase, EC 3.5.3.1) were determined in liver homogenates. Early growth of pigs was improved by increasing the protein level (12%) but not by adding the limiting amino acid lysine to the 12% diet. The cross sectional area of the Longissimus dorsi muscle at slaughter was increased by protein level (12%) or by adding lysine to the 12% protein diet. In contrast, subcutaneous fat was decreased only by increasing the dietary protein level. The activities of arginase, argininosuccinate synthetase and argininosuccinate lyase were increased in a linear fashion by increasing the dietary protein level. In contrast, adding lysine to the 12% protein diet did not increase urea cycle enzyme activities.