The medicinal plant, oregano (Origanum vulgare) (ORG) has been widely used for centuries in traditional medicine. Use of ORG is also developing in nutrition of fish and other domestic animals, since this plant has exhibited antioxidant, growth promoting and immunostimulating properties. To our knowledge, there are no information regarding the moderating effects of ORG on oxidative stress and toxicity induced by pesticides in fish. Thus, in the present study, we evaluated for the first time the moderating properties of ORG against toxicological effects of organophosphate pesticide, diazinon on growth and some components of fish metabolism. In non-diazinon-exposed fish, the triiodothyronine (T3) levels in serum and body weight index (BWI %) and specific growth rate (SGR)values were higher in fish supplemented with 2, 6 and 10 g ORG/kg diet compared to control (non-ORG-supplemented fish) and fish fed 14 g ORG/kg diet after 60 days feeding trial (P<0.05). Furthermore, the serum thyroxine (T4) levels increased in fish fed 6 and 10g ORG/kg diet compared to control and those supplemented with 2 and 14 g ORG/kg diet (P<0.05). Fish supplemented with 14 g/kg ORG diet showed the lowest BWI %, SGR and serum thyroid hormones compared to other groups (P<0.05). The lowest FCR values were observed in fish supplemented with 6 and 10 g ORG/kg diet (P<0.05). Also, the serum levels of cortisol and glucose and liver histology showed no significant alternations throughout the experiment (P>0.05). In diazinon exposed fish, thyroid hormones, BWI % and SGR significantly declined during 60 days exposure to diazinon in control and ORG-supplemented fish (P<0.05). However, these parameters were found to be higher in
