10.22376/ijpbs.2019.10.1.p1-12 Volume 6 Issue 1 2015 (January - March) The present study was aimed to investigate the effects of microcystin–LR (MCLR) on the blood electrolytes of lessThan i greaterThan Heteropneustes fossilis lessThan /i greaterThan . The fish were injected microcystin- LR (2.5 μg/25 g) intraperitoneally at the initiation of the experiment. Blood samples were collected and the serum calcium and phosphate levels were estimated at 1, 3, 5, 10, 15 days. There is no change in serum calcium level of microcystin-LR injected lessThan i greaterThan H. fossilis lessThan /i greaterThan at day 1. The levels indicate a progressive decrease from day 3 to day 5 which tend to recover from day 10 till the end of the experiment (day 15). The first perceivable change has been noticed in serum phosphate levels, which show a decrease at day 3. This decrease progresses till day 5. Thereafter, the levels show a tendency to become normal from day 10 to day 15. It is concluded from this study that microcystin–LR exposure to the fish lessThan i greaterThan H. fossilis lessThan /i greaterThan alters the blood calcium and phosphate levels of the fish, thus causing physiological disturbances which might adversely affect the normal vital functions, reproduction, growth rate and their survival in nature. CHANDRA PRAKASH, ABHISHEK KUMAR, MANIRAM PRASAD, SUNIL K. SRIVASTAV AND AJAI K. SRIVASTAV Microcystin- LR, Serum Calcium, Phosphate, Heteropneustes fossilis 1147-1153