Eight crossbred feeder steers were used in two consecutive N balance studies to investigate the effects of infectious bovine rhinotracheitis virus (IBRV) on N kinetics. Balance Study 1, which followed a 10-d acclimation phase, consisted of 7 d of sample collection referred to as the healthy phase (d -7 to d -1). Study 2, the IBRV-infected phase, began 2 d after a nasal IBRV challenge and continued for 6 d (d 2 to d 8). A stable isotope, [15N]-glycine, was used to determine N kinetics in both studies. Steers weighed 203 kg at the beginning of the study, 208 kg at IBRV infection and 194 kg at the end of study. Infection with IBRV increased (P less than .05) urinary N excretion from 17.9 to 31.5 g/d. Daily N balance was reduced (P less than .05) during infection from 21.2 to -3.3 g/d. Total serum proteins increased (P less than .05) during infection from 6.6 to 7.1 g/100 ml, the increase being predominantly in the alpha and gamma globulin fractions. Blood urea-N increased (P less than .05) during infection from 6.5 to 12.9 mg/100 ml. The urine excretion curve of the stable isotope and the N balance data indicated that IBRV infection increased N turnover and altered tissue utilization of N.