Background: The purpose of this study was to investigate the influence of parasympathetic nervous system on heart rate after the onset of exercise and immediate post exercise recovery. The heart rate is one of the most important cardiovascular parameter, which is very commonly used in sports to measure athletic performance. The parasympathetic nervous system is one of the major factors controlling the heart rate response due to exercise or any other stressful activity. It is also found that parasympathetic nervous system has a trainable characteristic. Hence the study focused on relating the heart rate response to the parasympathetic nervous system during the onset of exercise and immediate post exercise recovery. Methodology: This study followed an experimental approach to the problem through the analysis of archived data in the form of heart rate response to incremental workloads to volitional fatigue. The subjects in the archived data comprised of past research data measured during cycle ergometry testing to volitional fatigue. The demography of the subjects was determined via the archived data. The data consisted of 33 healthy male subjects between 19 and 47 years of age. The subjects were divided into two groups: very fit and moderately fit group, based on their VO2 max. The 50th percentile of the MaxVO2 was determined by using the median for the group. The median value obtained was 57.2 ml/kg/min. The subjects with the MaxVO2 above 57.2 ml/kg/min were grouped into a very fit category of the individuals, and the subjects with MaxVO2 below 57.2 ml/kg/min were grouped into moderately fit category. There were sixteen (n=16) subjects in the very fit group and seventeen (n=17) subjects in the moderately fit group. The outcome of the test was measured in heart rate. The parasympathetic withdrawal includes the heart rate data collected during the first three minutes of exercise. The parasympathetic reinfusion includes the heart rate data collected during the last three minutes of exercise. The statistical analysis comprised of correlation analysis and the t-test for independent samples. The results were: the correlation coefficient for very fit group parasympathetic withdrawal versus parasympathetic reinfusion is -0.81 at p < 0.05 and was found to be significant. The correlation coefficient for moderately fit group parasympathetic withdrawal versus parasympathetic reinfusion is 0.65 at p < 0.05 and was found to be non significant. A t-test for independent samples was used to determine the difference between the heart rate response due to parasympathetic withdrawal and heart rate response due to parasympathetic reinfusion for the very fit group of subjects, and the difference was found to be statistically significant at p value of 0.002. A t-test for independent samples was used to determine the difference between the heart rate response due to parasympathetic withdrawal and the heart rate response due to parasympathetic reinfusion for the moderately fit group of subjects, and the difference was found to be statistically significant at p value of 0.001. The conclusion is that there is significant association between heart rate response due parasympathetic withdrawal and heart rate response due to parasympathetic reinfusion for very fit group of subjects. But there is no significant association between heart rate response due to parasympathetic withdrawal and heart rate response due to parasympathetic reinfusion for moderately fit group of subjects. There exists a significant difference between the heart rate response due to parasympathetic withdrawal and heart rate response due to parasympathetic reinfusion in both very fit and moderately fit group of subjects.