Serum amino acid pattern, blood ammonia, and serum valproic acid (VPA) levels were estimated in 75 epileptic children with and without VPA therapy. The incidence of patients with hyperammonaemia (greater than 60 mumol/l) in patients treated with only VPA or with a combination of VPA and other antiepileptic drugs was 19% and 20% respectively. Hyperammonaemia was not observed in patients receiving other drugs. Elevated levels of serum glycine, alanine and serine were more common in epileptic patients treated with VPA alone and with VPA in combination with other drugs than in patients receiving other drugs. A significant positive correlation was found between the serum levels of glycine and alanine, of glycine and serine, and of serine and alanine. These findings may indicate a mitochondrial dysfunction caused by VPA.