M Heydari - 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 - Department of Environmental Sciences, Faculty of Natural Resources and Marine Sciences, Tarbiat Modares University, Noor, Mazandaran, Iran
S Hosseini - Department of Biology, School of Science, Alzahra University, Tehran, Iran
J Mahdavi - Expert of Shahid Motahary Cold Water Fishes Genetic and Breeding Research Center, Yasooj, Iran
A Movahedinia - Department of Marine Biology, Faculty of Marine Sciences, University of Mazandaran, Babolsar, Iran
Z Amini - Department of Marine Biology, Faculty of Marine Science, Khorramshahr University of Marine Science and Technology, Khorramshahr, Iran
S Rouzbehani - Graduated from Faculty of Biology Science, Shahid Beheshti University, Tehran, Iran

The purpose of this study was to determine the lethal concentration (LC۵۰-۹۶ 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 ۲۴ to ۹۶ hours. Twenty four hour lethal concentration (LC۵۰-۲۴ h) of malachite green on O. mykiss is ۳۲.۲۸ mg L-۱. Forty eight hour lethal concentration (LC۵۰-۴۸ h) of malachite green is ۱۶.۳۲ mg L-۱, and ۷۲-hour lethal concentration (LC۵۰-۷۲ h) of malachite green is ۲.۵۲ mg L-۱. All lethal concentrations at ۷۲ hours showed a clear reduction compared to ۲۴ and ۴۸ hours. The median lethal concentration of malachite green (LC۵۰-۹۶ h) of rainbow trout was ۰.۸۳ mg L-۱ during four consecutive days (۹۶ 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 LC۵۰-۹۶ 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.

Malachite Green, Median Lethal Concentration, Histopathology, Oncorhynchus mykiss