The efficacy of current therapeutic measures in orthostatic intolerance (OI) varies among patients and is oftentimes unsatisfactory. New approaches to alleviate symptoms of OI are therefore clearly needed. Recent reports have demonstrated that acetylcholinesterase inhibition is effective in the treatment of orthostatic hypotension with the presumed mechanism of enhancing sympathetic ganglionic transmission. Based on the hypothesis that acetylcholinesterase inhibition, by improving the safety factor of cholinergic transmission, will result in enhanced vascular adrenergic tone and a vagal shift in cardiac sympathovagal balance, we evaluated the role of acetylcholinesterase inhibition in the treatment of patients with OI. We monitored heart rate (HR), blood pressure, and indexes for cardiac output, end-diastolic volume, and systemic resistance continuously in 18 patients with OI during supine rest and during 5 minutes of 70 degrees head-up tilt before and 1 hour after oral administration of 60 mg pyridostigmine. Plasma catecholamines and baroreflex sensitivity were determined for the supine and upright position before and after medication. Patients scored orthostatic symptoms for both tilt studies. The excessive HR response to orthostatic stress was significantly blunted after pyridostigmine administration. HR was significantly lower in the supine and more so in the upright position. Baroreflex sensitivity in the upright position was significantly higher after pyridostigmine. Norepinephrine was increased in both supine and upright position. These changes were associated with significant improvement of orthostatic symptoms. We conclude that pyridostigmine improves orthostatic tolerance in patients with OI. Our findings support the suggested mechanisms of enhanced sympathetic ganglionic neurotransmission and a vagal shift in cardiac sympathovagal balance. Acetylcholinesterase inhibition could be a new useful concept in the treatment of OI.