Nitrogen losses in the urine are derived from amino acid oxidation, and the increased loss of urinary nitrogen during stress indicates accelerated amino acid oxidation. This study compared isoleucine and valine oxidation by traumatized rats with that by pair-fed control rats. Seventy rats received bilateral hind limb fractures and were fed an oral liquid diet ad libitum, and 70 healthy rats were pair-fed with the trauma group. Daily food intake, body weight, and 24-hr urinary nitrogen were monitored for each animal, and isoleucine and valine oxidation were measured on days 1 through 7 postinjury using five rats from each group for each amino acid. Amino acid oxidation was determined from the percentage of dose appearing in the breath during 4 hours following a single injection of C-14 labeled amino acid. Anesthesia had a pronounced effect on all parameters on day 1, and its effects were dissipated by day 2. Skeletal trauma produced elevated urinary nitrogen losses that lasted for 5 days and peaked on day 3. Valine and isoleucine oxidation were increased for 5 days, and the peak increase occurred on day 3 post-trauma. These data show that isoleucine and valine oxidation parallel excessive urinary nitrogen excretion after skeletal trauma and that isoleucine and valine, like leucine, contribute to the increased urinary losses after trauma.