The possibility that arginine vasopressin (AVP) is involved in the etiology of febrile convulsions was investigated by experiments on hyperthermia-induced convulsions in rats. Homozygous Brattleboro rats, which genetically lack AVP, and Long Evans rats, which were passively immunized by intracerebroventricular anti-AVP antiserum, either convulsed at higher body temperatures than untreated Long Evans rats or did not convulse at all. This indicates that a lack of AVP increases the threshold for the convulsions. High blood levels of AVP in hyperthermic convulsing rats compared to hyperthermic non-convulsive rats support the hypothesis that AVP may mediate febrile convulsions.