This study highlights the effect of extreme heat stress exposure (WBGT Index 36.24__ampersandsigndeg;C) on hematological variables while performing graded physical activity (sedentary, moderate and heavy). Total 30 participants were involved in the study and were randomly distributed into batches of 10 under three categories of physical activity. Volunteers performed physically strenuous work while being exposed to ambient heat stress (hot __ampersandsignndash;arid conditions). White Blood Corpuscles (WBC) count in heavy work group [506 kcal/ hr energy expenditure] showed significant increase from pre to post exposure (p __ampersandsignlt; 0.01). No other group showed significant change in WBC count. Haematocrit % (Hct %) was significantly increased among heavy work group (p __ampersandsignlt; 0.01) whereas Red Blood Corpuscles (RBC) count and Red Blood Cell Distribution Width (RDW) showed no significant increase. Blood viscosity showed increase among moderate work group [412kcal / hr energy expenditure] (p__ampersandsignlt;0.05) and heavy work group (p__ampersandsignlt; 0.001). These alterations indicate that participants exposed to high ambient temperature while doing graded physical activity underwent considerable hypo-hydration (sweat loss upto 4% of body weight) which got reflected in hematological parameters. This depletion of body fluid got reflected on blood viscosity and WBC count at a stage when the condition was not subjectively perceived. Data may be beneficial to identify such hypo hydration status before the condition is manifested visibly or symptomatically.