Crystalline amino acid diets containing different levels of total nitrogen (TN) and essential amino acid (EAA) nitrogen were evaluated in 3-week growth trials to identify a diet for use in determining the amino acid requirements of growing guinea pigs. Four levels of TN (2.5, 2.9, 3.6, and 4.3%) with constant EAA/TN ratios were compared by measuring weight gain, hemoglobin and hematocrit, carcass weight and nitrogen retention of 3-to 6-week-old male Hartley guinea pigs. Three levels of EAA nitrogen (1.7, 2.0, 2.4%), with TN levels constant at 3.6%, were also compared. A diet containing 30% casein + 0.3% arginine served as a purified control diet. Water and diets were provided ad libitum. The results of this study indicate that a diet containing 3.6% TN and 1.7% EAA nitrogen supported performance comparable to that of animals fed the casein diet. Growth rates for this amino acid diet and the casein diet both averaged 5.8 g/day.