en Toxicology and polution Toxicology and polution پژوهشي Original research papers <div style="text-align: justify;">The purpose of this study was to determine the lethal concentration (LC₅₀-96 h) of malachite green on rainbow trout and also the histology effects of this substance on the liver, gills and kidney. With increasing concentrations of malachite green, rainbow trout mortality increased during the period of 24 to 96 hours. Twenty four hour lethal concentration (LC₅₀-24 h) of malachite green on <em>O. mykiss</em> is 32.28 mg L^-1. Forty eight hour lethal concentration (LC₅₀-48 h) of malachite green is 16.32 mg L^-1, and 72-hour lethal concentration (LC₅₀-72 h) of malachite green is 2.52 mg L^-1. All lethal concentrations at 72 hours showed a clear reduction compared to 24 and 48 hours. The median lethal concentration of malachite green (LC₅₀-96 h) of rainbow trout was 0.83 mg L^-1 during four consecutive days (96 hours). On the other hand, malachite green showed destructive effects on liver, gills and kidney tissue of rainbow trout, and these changes were more intense with increasing concentration of malachite green. Therefore, due to the grading of toxicity is determined by the amount of LC₅₀-96 h and also observing tissue effects exposed to this substance, the malachite green is considered highly toxic to rainbow trout. For this reason, there are always concerns about the possibility of its transmission to consumers or humans; therefore, regarding to this matter that using this substance in the fish farms has been prohibited by Iran Veterinary Organization, it is necessary to remove malachite green from the list of drugs used in fish breeding and provide the other safe drugs.</div> Malachite Green, Median Lethal Concentration, Histopathology, Oncorhynchus mykiss 41 60 http://ijaah.ir/browse.php?a_code=A-10-382-1&slc_lang=en&sid=1 M Heydari heydari_mohsen84@yahoo.com 10031947532846002928 10031947532846002928 No Department of Aquatic Animal Health, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran, and Department of Pharmacognosy, Faculty of Pharmacy, Mazandaran University of Medical Sciences, Sari, Iran Z Yaghoobi Zeinab.yaghoobi@modares.ac.ir 10031947532846002929 10031947532846002929 No Department of Environmental Sciences, Faculty of Natural Resources and Marine Sciences, Tarbiat Modares University, Noor, Mazandaran, Iran S Hosseini saba.hoseyny@gmail.com 10031947532846002930 10031947532846002930 Yes Department of Biology, School of Science, Alzahra University, Tehran, Iran J Mahdavi mahdavejavad60@yahoo.com 10031947532846002931 10031947532846002931 No Expert of Shahid Motahary Cold Water Fishes Genetic and Breeding Research Center, Yasooj, Iran A Movahedinia amovahediniia@gmail.com 10031947532846002932 10031947532846002932 No Department of Marine Biology, Faculty of Marine Sciences, University of Mazandaran, Babolsar, Iran Z Amini Niusha_Amini1292@yahoo.com 10031947532846002933 10031947532846002933 No Department of Marine Biology, Faculty of Marine Science, Khorramshahr University of Marine Science and Technology, Khorramshahr, Iran S Rouzbehani 10031947532846002934 10031947532846002934 No Graduated from Faculty of Biology Science, Shahid Beheshti University, Tehran, Iran