Background: In gastrointestinal system, parasympathetic tone causes stimulation of all gastric glands and enhances Hydrochloric acid (HCl) secretion which predisposes to peptic ulcer. Aim: To study the role of ABO blood group systems on parasympathetic activity in healthy volunteers. Increased parasympathetic activity in any of these groups will cause increased Hydrochloric acid secretion in gastric juice contributing to increase incidence of peptic ulcer in that group. Methodology: This study was conducted in Department of Physiology, Saveetha Medical college, Chennai. Total of 160 healthy adults of both genders (18-24 years) were included in the study and were divided equally into 4 groups based on their blood groups (ABO system). After obtaining the written consent from the subjects, Heart Rate Variability (HRV) was analysed by frequency domain and time domain methods in all groups using digital physiograph. Results: Statistical analysis done by One way analysis of Variance (One way ANOVA), showed a statistically significant difference (P __ampersandsignlt; 0.001) in HRV parameters among the 4 ABO blood groups. In frequency domain measures, an increase in High frequency HF in normalised Units and decrease in Low frequency (LF) in normalised Units, and the ratio of LF to HF (LF/HF) is decreased in A and O blood groups. In the time domain measures, increase in RR intervals, Standard deviations of all NN intervals (SDNN), Square root of mean of squares of the differences between adjacent NN intervals (RMSSD), Percentage of differences between adjacent NN intervals (pNN50) in A and O compared to B and AB blood group individuals. Conclusion: The present study showed that the A blood group (ABO system) have higher parasympathetic tone when compared to the other blood group systems..