Background: Previous studies have been reported on the toxicity of Microcystin-LR, which is produced by cyanobacterial growth in fish, such as Heteropneustes fossilis (H. fossilis). However, no studies have been conducted on the effects of Microcystin-LR on the prolactin cells of H. fossilis. Methods: H. fossilis fish were intraperitoneally injected with Microcystin-LR (2. 5μ 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's tetrachrome and Heidenhan's azan techniques, and examined under light microscopy. Results: 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. Conclusions: Our results indicated that the serum calcium levels and prolactin cells were altered after exposure to microcystin-LR.