AbstractBackground: Autonomic nerve function impairment is related to development of Irritable Bowel Syndrome (IBS). Time domain measures of Heart rate variability (HRV) is a useful tool to measure autonomic nerve function activity.Objective: To assess autonomic nerve function activity by time domain measures of heart rate variability in patients with Irritable Bowel Syndrome.Methods: This cross sectional study was conducted in the Department of Physiology, Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka in 2011. Ninety patients aged 20-50 years of both sex with Irritable bowel syndrome were included in the study group. They were collected from the OPD of Gastroenterology in BSMMU. Age and sex matched 30 apparently healthy subjects served as control. For assessing HRV by time domain method, Mean heart rate Mean R-R interval, Max/Min R-R interval, SDNN, RMSSD, PNN50%, NN50% were recorded by a digital Polyrite. ANOVA, independent sample t-test and Pearson’s correlation coefficient tests were performed as applicable.Results: Mean heart rate were significantly higher and Mean R-R interval, Max/Min R-R interval, SDNN, RMSSD, PNN50%, NN50% were significantly lower in IBS groups compared to those of control. Correlation analysis showed negative correlations of SDNN, RMSSD, PNN50%, NN50% with duration of disease.Conclusion: This study concludes that parasympathetic activity was reduced in patients of IBS. In addition, decreased vagal modulation is inversely related to the duration IBS.