Electrodermal activity (EDA) increases during arousal, emotional stimuli or thermoregulatory sweating and is sometimes apparent in unfiltered ECG recordings. We hypothesized that changes in EDA precede any change in blood pressure (BP) or cerebral blood velocity (CBV) observed at syncope. Data from 70 patients referred for recurrent syncope were retrospectively analyzed. 400Hz continuous waveforms were recorded of BP (Finapres), CBV (transcranial Doppler), expired CO(2) (PCO(2)) and ECG at rest and during 80 degrees head-up tilt (HUT). Two independent investigators determined the onset and termination of EDA in relation to syncope using the raw and a low pass filtered ECG trace. Of the 53 patients who experienced syncope during HUT, 33 (62 %) increased EDA 257 +/- 357s prior to syncope that continued for 75 +/- 73s following syncope. Seven patients (13%) had EDA only after syncope for 208 +/- 213s. In 13 patients (25 %), EDA was not detectable. In most cases, EDA preceded any change in BP, PCO(2) or CBV and persisted past the hemodynamic recovery following syncope. Although variable, EDA may be an objective correlate to the clinical observation that patients' symptoms precede any measurable change in cerebral perfusion. The actual relationship between EDA and symptoms of presyncope requires further investigation.