Metronidazole (MTZ) is commonly used as an antibiotic for enterozoa; however, its side-effects have not been fully characterized. In this study, the toxic effect of a dose and time-dependent MTZ application on morphology of the intestinal tissue of Oncorhynchus inykiss was examined. Fish were exposed to 5, 10, and 20 mg/L of MTZ in the aquariums for 2, 4 and 8 days. Samples taken from the jejunum region of the small intestine were for Gomori Trichrome, NOS and TUNEL. A slight toxic effect that started with a dosage of 5 mg/L for 2 days reached the maximum level with a dosage of 20 mg/L for 8 days. Erosion, oedema, inflammation, disintegration in the epithelium, and necrosis were detected at the tips of the villi. NOS staining had increased a great deal. Separately, a parallel TUNEL staining was monitored in the process of NOS staining practices. The toxic effect that increased in proportion to the dosage and period had occurred due to oxidative damage and that the cells had been induced to die through apoptosis. It was thought that this toxic effect could be of significance because of food chain and needed to be investigated by means of advanced techniques
