The neonatal hyperammonemia is the pathological condition that occurs during the neonatal period; it is characterized by the increased content of the free ammonium ions in the blood, and it causes the severe neurological disorders. The hyperammonemia in the newborns is one of the manifestations of a wide range of both primary (hereditary) and secondary metabolic disorders. Depending on the specific cause, the hyperammonemia in the neonatal period can be of the persistent or transient nature. As a rule, the clinical signs of this condition are nonspecific. The neurological disorders of the varying severity are noted, as follows: CNS (central nervous system) excitement or depression syndrome, episodes of apnea, diffuse muscular hypotonia, convulsive disorder and coma. The hyperammonemia can be accompanied by the respiratory disorders, and it can cause the development of the multiple organ failure that resembles the clinical symptom complex of the sepsis. The severity of brain damage correlates with the degree of increase in the ammonia concentration and hyperammonemia duration. Early diagnosis of the hyperammonemia allows to save the child's life, to prevent the severe neurological consequences and to reduce the risk of disability. Moreover, the identification of the hereditary metabolic diseases accompanied by the hyperammonemia determines the necessity to carry out the genetic counselling of the family, as well as the prenatal and preimplantation genetic diagnosis.