Urea cycle function was evaluated in liver obtained from six patients with Reye's syndrome and from five control subjects. Reye's syndrome patients demonstrated normal activities for the extramitochondrial portion of the urea cycle, but showed marked abnormalities of the mitochondrial enzymes, i.e., carbamyl phosphate synthetase (CPS) and ornithine transcarbamylase (OTC) (Tables 2,3). CPS activity was reduced to less than 15% of control values in all four patients from whom tissues was obtained during the first 72 hr after the onset of encephalopathy. Two patents from whom tissue was not obtained until after 9 days of symptoms showed no reduction in CPS activity. The OTC activity was also reduced (3-67% of control values) in the four patients from whom tissue was obtained early in the illness. In addition, greater than 60% reduction in Vmax and Km for carbamyl phosphate was noted in all four patients in whom sample size permitted kinetic analysis, including both patients in whom CPS and OTC activity were not markedly reduced. The same kinetic abnormality as well as decreased CPS activity were experimentally produced in normal rate liver incubated in the presence of 1.0 mM 4-pentenoic acid, a short chain fatty acid and known hepatic mitochondrial toxin (Table 4).