Hypohydration or dehydration is the condition in which body water level falls below normal.The effects of hypohydration on thermoregulatory, cardiovascular and central nervous function become increasingly detrimental with degree of severity. Soldiers, athletes and commercial kitchen workers etc. are at a greater risk. Physical activity can result in 1-8% body mass loss.Dehydration level of 2% body mass is a threshold above which measurable changes in various physiological parameters are observed. Previous studies on hypohydration have focused on a very few and isolated parameters. The current study has a broader scope and gives and intergrated picture of the effect of 4% body mass loss due to exertional heat stress induced hypohydration in humans. The study was approved from Institutional human ethical committee. Six healthy military personnel (age:25±4 years , height:172±4cm, weight:66±3 Kg) volunteered to participate in the study. They were informed about the purpose of the study and possible risks and signed a consent form.Volunteers were made to drink appropriate amount of water to attain euhydrated state one hour before the experiment. They were made to perform free hand exercise at 45C and 30% RH in human climatic chamber, till 4% body weight reduction was observed. Various parameters such as Heart Rate Variability, cardiac output and stroke volume, exhaled Nitric Oxide, Bio Impedance, core body and skin mean temperature were measured before and after the heat exposure. Blood samples were drawn and plasma was obtained and stored at -80C for biochemical analysis. A significant increase in the level of hormones such as cortisol (1.9± .02nMol/L vs 2.2±.02 nMol/L, p<0.001), angiotensin II (0.123±0.02ng/L vs .317±0.06 ng/L, p<0.01), Antidieuretic hormone(56.5±12.9ng/L vs 123.3±24.4ng/L, p<0.01) was observed using paired T-test in plasma samples collected post 4% hypohydration compared to control. Also, a significant increase was observed in core body temperature (36.6 ± 0.1C vs 37.5 ± 0.1C, p<0.001), skin mean temperature (35.1±0.1C vs 36.01 ±0.1C, p<0.001), total body water (48.99 ± 3L vs 44.83 ± 3L, p<0.001), extracellular water (20.90 ±2L vs 19.64 ± 2L, p<0.001), the LF:HF ratio(6.20 ± 3 vs 9.04± 4 , p<0.001) post 4% hypohydration as compared to control using paired T-test. No significant change was observed in the intracellular water volume, exhaled nitric oxide, cardiac output and stroke volume post 4% hypohydration as compared to control. This is a comprehensive, multi-parametric study, with an aim to analyze the linkage between physiological and biochemical responses generated upon exertional heat stress induced hypohydration. Further in-depth studies are required to attain this goal. We plan to carry out experiments using Rat as model system to elucidate the exact mechanisms and pertinent pathways, which underlie hypohydration.