The authors investigated whether heart-rate variability can serve as a device for real-time quantitative measurement of hypnotic depth. This study compared the continuous self-rated hypnotic depth (SRHD) of 10 volunteers with heart rate, amplitude, and frequency changes from a time-frequency analysis of heart-rate variability (HRV). The authors found significant linear relationships between SRHD and the high-frequency (HF) component of HRV. Specifically, SRHD was correlated negatively with the frequency of the HF component and positively with the amplitude of the HF component. Unexpectedly, the average temporal trend in SRHD fit well (R(2) = .99) to the step response of a first-order system with a 4-minute time constant. The findings suggest that the reactivity of the parasympathetic branch of the autonomic nervous system reflected in HRV could become part of a real-time, quantitative measure of hypnotic depth.