We analyzed heart variability (HRV) response to tilt in 35 patients with a history of neurocardiogenic syncope and in 8 normal volunteers. Frequency domain examination was performed using power spectral analysis of RR variability during resting supine position and during the 256 beats preceding the onset of syncope or completion of 60 min of upright tilt to 70 degrees. Both low (0.05-0.15 Hz) and high frequency (0.15-0.4 Hz) spectral components of HRV increased markedly in all groups as a result of tilting. Statistically significant differences were noted between the patient groups at completion of upright tilting. However, these differences in spectral components between groups were of small magnitude when compared to the overall increase in spectral power occurring in all groups as a result of the tilt and are difficult to correlate clinically with the appearance, or lack of appearance of tilt-induced syncope. Power spectral analysis of HRV, as presently performed, appears to lack discriminative power to detect the rapid and marked changes in sympathovagal modulation known to occur during tilt test in patients with neurocardiogenic syncope.