Vasopressin (aVP) at low concentrations functions as an antidiuretic hormone and has vasoconstrictive effects. To investigate the possible role of aVP in the pathogenesis of migraine, six patients with a history of induced migraine were given 100 g chocolate, and blood samples for plasma aVP were taken before ingestion and every hour for 4 h. In one patient who presented with severe headache and nausea the base-line plasma aVP concentration was 15.2 pg/ml; it fell to 3.2 pg/ml at 2 h before rising to 10 pg/ml at 3 h and 4 h as the symptoms worsened. In the five patients with moderate or no headache plasma aVP concentrations remained in the normal range (less than 3 pg/ml) throughout. The results suggest that aVP does not have a role in the aetiology of migraine. The possibility exists that during severe attacks of nausea there is release of aVP, which may be responsible for the facial pallor, antidiuresis, and coagulation abnormalities occasionally observed in migraine.