en عمومى General پژوهشي Research <strong>Background</strong>: Previous studies have been reported on the toxicity of Microcystin-LR, which is produced by cyanobacterial growth in fish, such as <em>Heteropneustes fossilis</em> (<em>H. fossilis</em>). However, no studies have been conducted on the effects of Microcystin-LR on the prolactin cells of <em>H. fossilis</em>.<br> <strong>Methods</strong>:&nbsp;<em>H. fossilis</em> fish were intraperitoneally injected with Microcystin-LR (2.5&mu;g/25g) and sacrificed after 1, 3, 5, 10 and 15 days. The blood samples were analyzed for the calcium levels. Histological slides of the pituitary prolactin cells were stained with Herlant&rsquo;s tetrachrome and Heidenhan&rsquo;s azan techniques, and examined under light microscopy.<br> <strong>Results</strong>:&nbsp; &nbsp;The prolactin cells exposed to Microcystin-LR exhibited no structural changes on day 1. However, hyperactive prolactin cells exhibited cellular degranulation from day 3 to day 5. On days 10 and 15, degenerated and vacuolated prolactin cells were also observed. The nuclear volume of prolactin cells exposed to MC-LR increased progressively from day 3 to day 10 but on day 15 the nuclear volume returned to normal. The serum calcium level of MC-LR injected specimens showed hypocalcemia from day 3 to day 10. On day 15, the level became normal.<br> <strong>Conclusions</strong>: Our results indicated that the serum calcium levels and prolactin cells were altered after exposure to microcystin-LR. Blood Calcium Level, Heteropneustes Fossilis, Microcystin-LR, Prolactin Cells, Prolactinemia 17 20 http://ijt.arakmu.ac.ir/browse.php?a_code=A-10-562-1&slc_lang=en&sid=1 Chandra Prakash 100319475328460011519 100319475328460011519 No Department of Zoology, DDU Gorakhpur University, Gorakhpur, India. Sunil Kumar Srivastav sunilsrivastav@hotmail.com 100319475328460011520 100319475328460011520 Yes Department of Zoology, DDU Gorakhpur University, Gorakhpur, India.