Influence of mental stress on cardiovascular function differs from individual to individual, based on a variety of inherent factors (1). . Some people show characteristically greater cardiovascular reactivity than others to behaviourally stressful situations. Cardiovascular reactivity reflects the functional status of autonomic nervous system (2). The first year medical students are stressed by emotional factors and also have less physical activity The current study was undertaken to establish a basal autonomic status of adolescent south east Asian students and to find out the changes in cardiovascular parameters and their correlation with physical fitness and body composition. 50 healthy medical students of the age group of 17-19 years were recruited for the study. Cardiorespiratory fitness was assessed by Harvard Step Test and Body Mass Index and body fat% were measured by Body composition analyzer. Basal autonomic status was studied by calculating changes in heart rate under orthostatic stress from electrocardiographic records and measurement of blood pressure by sphygmomanometer (3). All cardiovascular parameters were recorded before and after application of mental stress by means of public speaking in front of peer group. All data were analysed statistically by using SPSS version 19.0. Informed consent was taken from all subjects and permission of the institutional review board was obtained. Results showed that students with low BMI had a 30:15 ratio of 1.2[0.16] while those with high BMI had a ratio of 1.3[0.28]. Upon application of mental stress, the rate of change of heart rate also varied according to the BMI and Body fat%. Rate of change of heart rate was higher [39.5 +/- 1.7; 32.8+/-6.05] in both the high fat group and low fat group as compared to the normal, high fat group showing a bigger change. Rate of change of systolic pressure showed a similar trend while diastolic pressure change was lower in the high fat group. Subjects with low physical fitness had a poor parasympathetic control which may indicate that they are more vulnerable to break down under mental stress. The data suggested that autonomic reactivity to mental stress is correlated with basal physical fitness level of subjects and is also dependent on their body composition. Basal fitness level did not show any relationship with body composition but both low fitness level and high body fat % were found to show poor autonomic regulation of cardiovascular system. In such individuals, mental stress can cause higher alteration of heart rate and systolic blood pressure due to altered autonomic regulation.