Unlike adults, neonates tolerate high thoracic spinal anesthesia with minimal changes in heart rate (HR) and arterial blood pressure. To examine the potential autonomic regulatory mechanisms which may account for these findings, the relation between short-term heart rate variability (HRV) and respiratory activity was analyzed in a group of eight ASA grade II former premature infants undergoing high thoracic spinal anesthesia for inguinal hernia repairs. Quantitative measures of sympathetic (As) and parasympathetic (Ap) modulation of HR were derived. HR, arterial blood pressure, and a calibrated respiratory signal were recorded during 4.4-min stable epochs in eight subjects 1) preoperatively, 2) postincision after high thoracic spinal anesthesia, and 3) during an active sleep state in the postoperative period. Power spectral analysis of HRV and respiratory power yielded measures of low-frequency power (LFP: 0.02-0.15 Hz) and high-frequency power (HFP: 0.15-0.8 Hz). Transfer function analysis between respiratory activity and HR were used to quantify As and Ap. All subjects had successful high thoracic spinal anesthesia with highest levels ranging from C7-T4. Mean HR, blood pressure, and respiratory power did not change significantly with high thoracic spinal anesthesia. LFP and HFP both decreased significantly, whereas the LFP/HFP ratio remained stable. Group mean As and Ap both decreased, but the changes were not significant. Despite overall cardiovascular stability, HRV decreased with high thoracic spinal anesthesia, but the balance between LFP and HFP remained stable, suggesting that the reflex response to high thoracic spinal anesthesia was predominantly diminished parasympathetic modulation of cardiac function. The expected decrease in HR and blood pressure from the sympatholysis which results from high thoracic spinal anesthesia were apparently offset by withdrawal of cardiac vagal activity.