Although patients diagnosed as having migraine, have been studied extensively, there is no definite proof of a primary biochemical abnormality which is of significance in the pathogenesis of migraine. Several authors have suggested that increased serum ammonia levels may play a decisive role in the pathogenesis of migraine. This possibility was first put forth by Russell, who studied a single family with ornithine carbamyl transferase deficiency and 8 juveniles with migraine, RUSSELL'S finding was supported by a preliminary study carried out by one of the authors in which hyperammo-naemia was found in 22 adults during migraine attacks. In order to etknd these earlier findings, serum ammonia levels were determined in 35 adults with either classic or complicated migraine and in 20 healthy adults. No significant difference was found between the patients (44 microgram/100 ml +/- 13.9) and the controls (74 microgram/100 ml +/- 9.5). Ammonia level was determined in 10 patients within two hours of the onset of an acute migraine and was unchanged (65 microgram/100 ml). This study suggests that alterations in serum ammonia level are not important in the pathogenesis of migraine in adults.