The medicinal plant, Ginkgo biloba (GB) has been used for centuries in traditional medicine to
improve health in human. In this study, we investigated for the first time the protective effects of
GB against the toxic effects of the organophosphate pesticide (PC), diazinon on metabolic hormones, plasma glucose, liver histology and growth parameters of rainbow trout, Oncorhynchus
mykiss. In non-diazinon-exposed fish, groups supplemented with 1 and 2 g GB/kg diet showed
higher levels of plasma thyroid hormones [(Triiodothyronine (T3) and thyroxine (T4)], body weight
index (BWI %) and specific growth rate (SGR %) compared to control (non-GB-supplemented fish)
(p< .05). The supplementation of fish with 4 g GB/kg diet decreased the BWI %, SGR %, and
plasma thyroid hormones (p < .05). FCR values significantly decreased in fish supplemented with
1 and 2 g GB/kg diet (p < .05). Also, no alternations were found in plasma levels of cortisol and
glucose and liver histology during 60 d experiment (p> .05). In diazinon-exposed fish, thyroid hormones, BWI %, and SGR % significantly decreased during exposure period in all experimental
groups (p < .05). FCR values significantly increased in control and fish supplemented with 0.5 and
4 g GB/kg diet (p < .05). No significant changes were observed in FCR values of fish fed 1 and
2 g GB/kg diet (p> .05). The cortisol levels significantly elevated during exposure period in control
and groups fed 0.5, 1, and 4 g GB/kg diet, while this hormone remained unchanged in fish supplemented with 2 g GB/kg diet (p > .05). The plasma levels of glucose significantly elevated in fish
supplemented with 2 g GB/kg diet (p< .5). Based on histological observations, the severity of liver
damages was lower in GB-supplemented fish compared to non-supplemented ones (p < .5). The
lowest liver lesions were observed in fish supplemented with 1 and 2 g GB/kg diet (p< .5). In conclusion, the findings of this study showed that GB at optimum dietary levels (1–2 g GB/
