Investigations were made on the blood of Mastacembelus armatus exposed to heavy metals (Cu, Ni, Fe, Co, Mn, Cr and Zn). The study reveals the contamination of Harduaganj Reservoir with heavy metals. The heavy metal content in reservoir water were in the order of Fe > Cu > Zn > Mn > Ni > Co > Cr. The values for the heavy metals such as Fe (8.71 mgL-1), Ni (0.12 mgL-1), and Mn (0.21 mgL-1) were higher than the limits suggested by UNEPGEMS. Whereas the concentration of heavy metals Cu, Cr and Zn were within the limits proposed. Mastacembelus armatus thriving in the reservoir water exhibited the influence of contamination.The exposed fish showed the significant decrease in total RBC count (2.16 x106mm-3), significant increase in total WBC count (3.84 x 103 mm3) and insignificant decrease in Hb (9.48 gdL-1) and when compared to control.