We have performed localized in vivo proton magnetic resonance spectroscopy on two females with ornithine carbamoyl transferase deficiency during episodes of acute hyperammonemic encephalopathy with focal neurologic abnormalities. Spectra obtained from 2 x 2 x 2 cm cubic volumes at relatively long (135-ms) echo times contain additional signals that are characteristic of glutamine and indicate that glutamine is present in very high concentrations in the brain. The findings are consistent with the hypothesis that intracerebral accumulation of glutamine contributes to the encephalopathy associated with hyperammonemia. In one of the children, spectra obtained after treatment showed a marked decrease in the glutamine signals.