The initial clinical symptoms of ornithine carbamoyl transferase deficiency depend on the age of onset. Respiratory distress on the first day of life does not allow exclusion of OCT deficiency in the individual patient. The acid-base status is not useful as a discriminant between urea-cycle disorders and organic acidurias. Beyond the neonatal age, a second period of increased risk for often lethal hyperammonaemic crises is found between 12 and 15 years of age. For definite diagnosis (pre- and postnatal) of heterozygotes the quantity of tissue obtained should be sufficient to obtain a representative sample for a mosaic structure. Experimental work gives some clues for the interpretation of reversible symptoms of hyperammonaemia. The increased transport of tryptophan at the blood-brain barrier in presence of increased glutamine concentration in tissue appears to depend on intact gammaglutamyl transpeptidase in brain microvessels and involves at least in part the L-carrier. Animal research on the mechanisms leading to irreversible damage in hyperammonaemia should be encouraged in order to define reliable predictive criteria for clinical decisions.