Ornithine transcarbamylase deficiency is the most common inherited urea cycle disorder, and is transmitted as an X-linked trait. The metabolic derangements produced by the classic form of deficiency result in vomitting, lethargy leading to coma, convulsions and if not treaed, death. Fluctuating concentrations of ammonia, glutamine, and other excitoxic amino acid result in a chronic or episodically recurring encephalopathy, and lead to a loss of cholinergic neurons and seem to play the main part in the pathophysiology of mental retardation and seizures which may be seen in ornithine transcarbamylase deficiency. In this report we describe a 16 year old of heterozygous female patient first presented with protein intolerance, attacks f vomitting, and signs of mental retardation in early childhood, and occurence of complex-partial seizures after initiation of quetiapine theraphy for behavioral symptoms of mental retardation. Af ter withdrawal of quetiapine seizures dissappeared. Antipsychotics are known to induce epileptic seizures and changes in EEg patterns, and may worsen a preexist ing epileptic vulnerability risks in children with urea cycle disorders along with mental retardation. When treating behavioral symptoms of mental retardation in this patient group we should be aware of antipsychotics induced epileptic sezizures.