In a patient with tetanus we tested the hypothesis that the hyperadrenergic cardiovascular instability might be due to impairment of the baroreceptor reflex by the tetanus toxin. Baroreflex sensitivity assessed with the phenylephrine method was found to be normal. Changes in arterial pressure correlated inversely with relative changes in plasma volume but not with plasma catecholamine levels. There were both extreme hypo- and hyperadrenergic episodes. We conclude that sympathetic overactivity in tetauns temporarily overrules a functionally intact baroreflex leading to severe blood pressure instability with episodes of hypertension.