Background: Sedentary living is responsible for about one-third of deaths due to coronary heart disease, hypertension and diabetes for which physical inactivity is an established causal factor. Aims & objectives: To study the effect of high, moderate and low intensity exercises on heart rate variability. Material and methods : The study comprised of randomly selected 65 1st year students (boys), aged 17-21 years. BMI of students was in range of 17-34.The students weredivided into 3 groups byusing W.H.O. Standard criteria for grading of physical activity. GROUP I(n=36)- Students who played sportslike cricket, badminton, soccer,basketball, swimming and jogging for >5 hrs/week for 8 weeks were categorised as performing vigorous activity/ grade-1. GROUP II(n=22) – Students who did brisk walking, cycling, played volleyball and table tennis for 3-5 hrs/week for 8 weeks were categorised as performing moderate activity or grade-2. GROUP III(n=7) - Students who did weight lifting, situps , pushups and walking for <3 hrs/week for 8 weeks were categorised as performing low activityor grade-3.Results:Heart rate variability has proved to be more sensitive tool for the detection ofautonomic balance than meanheart rate (HR). The Studyshowed significant correlationofmoderate and vigorous activity with heart rate variability.Due to this physical activity there was predominace of parasympathetic activity over the sympathetic activity resulting in decrease heart rate. Conclusion: Vigorous andmoderate physicalactivityare considered better than low physical activityto remain fit and healthy. It plays a vitalrole in reducing risk of coronaryarterydisease,diabetes and hypertensionin coming years.