The effect of unilateral stellate ganglion block on cardiovascular regulation remains controversial, so the present study used power spectral analysis of heart rate variability to investigate its effect on the autonomic neural control of the heart. In 20 young healthy volunteers (mean age: 25 years), heart rate variability was determined before and after unilateral stellate ganglion block (right side 11, left side 9) using 8 ml of 1% mepivacaine during supine rest. Using autoregressive spectrum analysis, power spectra were quantified by measuring the area in 3 frequency bands: high-frequency power (lnHF, parasympathetic influence) from 0.15 to 0.40 Hz, low-frequency power (lnLF, predominantly sympathetic influence) from 0.04 to 0.15 Hz, and total-frequency power (lnTF) less than 0.40 Hz. Right stellate ganglion block decreased not only the lnLF component from 6.55±0.84 to 5.77±0.47 but also the lnHF component from 4.40 ±0.95 to 3.42±1.12 (p<0.05). In contrast, left stellate ganglion block changed neither the lnLF nor the lnHF component. The lnTF component was also decreased significantly by right stellate ganglion block from 7.80±0.95 to 7.01±0.36 (p<0.05), but was unchanged following left stellate ganglion block. Neither right nor left stellate ganglion block induced any significant change in both the RR and corrected QT intervals. However, changes in the RR interval induced by right stellate ganglion block showed significant positive correlation with changes in lnHF (p<0.005) and lnTF (p<0.05). These results suggest that (1) autonomic innervation to the sinus node is mainly through the right-sided stellate ganglion, (2) pharmacological right-sided stellate ganglion block may attenuate not only sympathetic but also parasympathetic activity and (3) following right stellate ganglion block the decrease in both the sympathetic and parasympathetic influence on the sinus node may inconsistently counterbalance and change the RR interval. (Jpn Circ J 1999; 63: 854 - 858)