Purpose: The Newborn Infant Parasympathetic Evaluation (NIPE) monitor is an objective, non-invasive tool for the assessment of pain in children <2 years-of-age. The aim of this study was to evaluate the ability of the NIPE monitor to detect pain in neonates and infants during surgery under general anaesthesia. Methodology: This prospective observational study included neonates and infants undergoing elective inguinal hernia repair under general anaesthesia with a caudal block. The NIPE monitor was connected to the electrocardiogram monitor with continuous monitoring performed intraoperatively. The NIPE index was recorded prior to and during a 120 second interval following a nociceptive event. The nociceptive events evaluated were endotracheal intubation, venous cannulation, caudal block, and skin incision. The NIPE index ranges from 0-100, where lower values indicate greater levels of pain. P values <0.05 were considered significant. Data are reported as mean [95% CI]. Results: There were 40 infants recruited and a total of 115 events were analysed. All nociceptive stimuli caused a decrease in the NIPE index, indicating an overall ability to detect pain. The NIPE index decreased by 26.1% [9.9 - 42.3%] after intubation (p<0.0001), decreased by 12.1% [4.5 - 19.7%] after venous cannulation (p=0.004), decreased by 19.4% [10.9 - 27.9%] after caudal block (p<0.0001), and decreased by 18.3% [9.5 - 27.1%] after skin incision (p=0.0002). Conclusion: NIPE monitor is effective in detecting intraoperative painful stimuli under general anaesthesia. Further research is required to investigate other clinical contexts in which this monitor can be practically used.