Hyperornithinemia associated with hyperammonemia and homocitrullinuria (HHH syndrome) has been suggested to be due to a defect of ornithine transport across the inner mitochondria membrane. Preliminary experiments using the digitonin technique for the separation of cytosol and peller fraction applied to fibroblasts of 3 HHH syndrome patients have provided evidence for such a transport defect. Such a transport defect could be due to either a defect in the carrier specific for ornithine or due to an increase in intramitochondrial pH. The kinetics of uptake of ornithine by fibroblasts of normal controls and of HHH syndrome patients will be measured, as well as the uptake by mitochondria. The intracellular pH of fibroblasts and the intramitochondrial pH will be measured. To gain some insight into the mechanism of homocitrulline formation, the uptake of lysine by mitochondria, the kinetics of ornithine transcarbamylase with lysine as substrate and the conditions where lysine can give rise to the formation of homocitrulline in isolated hepatocytes will be studied.