Symptomatic transient neonatal hyperammonaemia is a dramatic condition which calls for comprehensive treatment. A boy with a birth weight of 1800 g and length of 42 cm was born during the 34th week of gestation and had no signs of asphyxia. During the first hours he developed respiratory stress which improved after mechanical ventilation. On the third day alteration of consciousness developed and generalized tonic spasms. Hyperammonaemia (282.90 mmol/l) responded well to metabolic manipulation and a rapid drop of the ammonia blood level was achieved without the use of elimination methods. Paroxysms disappeared after antioedematous treatment of the brain and phenobarbital administration. The deterioration left permanent neurological changes: at the age of 4 years the patient is mentally retarded and quadruspastic. During 1997 - 2000 hyperammonaemia was investigated in premature neonates with slight behavioural deviations during their first weeks of life. Plasma ammonia values of 50.0 - 80.0 mmol/l were found in 55% of children without clinical symptoms of metabolic instability.

        Key words: transient neonatal hyperammonaemia, asymptomatic form, metabolic manipulation