Orthostatic tachycardia is a poorly understood syndrome in which patients develop dizziness, diaphoresis, or palpitations upon shifting from the supine to the upright posture. The present study was performed to determine whether autonomic neuropathy might be present in these patients, and whether the abnormal hemodynamic response to standing might be the result of failure of reflex vasoconstriction. We measured autonomic function in 9 patients with idiopathic orthostatic tachycardia and 2 patients with orthostatic tachycardia and insulin-dependent diabetes mellitus and compared them to 33 age-matched controls. Although most patients with orthostatic tachycardia had normal vasomotor reflexes and normal surface potential amplitudes, the latency of the autonomic response, a measure of sympathetic nerve conduction velocity, was prolonged in the soles (2.44 ± 0.08 s in patients with idiopathic orthostatic tachycardia vs. 2.12 ± 0.04 s in controls; P < 0.005). In 6 of 9 patients, however, the latencies were within the normal range. Autonomic surface potentials were absent in 1 diabetic patient with orthostatic tachycardia; the latency of the response in the feet was greatly prolonged (2.95 s) in the second patient. We also assessed the response of orthostatic tachycardia patients to octreotide and dihydroergotamine, which are known to have a pressor effect in patients with recognized forms of autonomic neuropathy. These agents, in combination, suppressed orthostatic tachycardia (from 116 ± 7 to 89 ± 6 beats/min; P < 0.001) in patients with this syndrome. In summary, our data indicate that evidence of autonomic dysfunction is present in only a minority of patients with orthostatic tachycardia. Nevertheless, administration of the vasoconstrictor drugs dihydroergotamine and octreotide can prevent the abnormal hemodynamic response to the upright posture shown by patients with this syndrome.