BACKGROUND: Safflower plant can be used in fish due to its antioxidant properties. In the present study, the side effects of intraperitoneal injection of safflower extract in rainbow trout have been investigated. OBJECTIVES: The effect of the intraperitoneal injection of Safflower (Carthamus tinctorius) extract on Aspartate aminotransferase, Alanine aminotransferase and Alkaline phosphatase as tissue damage indicators and also the histopathologic analysis of kidney and liver tissues in rainbow trout (Oncorhynchus mykiss) have been investigated. METHODS: To this end, rainbow trout with an average weight of 100 ±5 gr were supplied and intraperitoneally injected with different levels of Safflower extract. In this regard, one negative control group (with no injection), one positive control group (injected with 0.2 ml normal saline) and three treatment groups (injected with 50, 100 and 200 mg/kgbw of safflower extract, respectively) were considered. Blood samples were taken on the third, seventh and tenth days after injection, in order to isolate blood serums and analyze the ALP, ALATA and ASAT activities. Kidney and liver tissue samples were also taken on the seventh-day post injection. RESULTS: The levels of ALP, ALATA and ASAT activities significantly increased in all treatment groups that received safflower extract compared to control groups in all samples (sig<0.05). In histological analysis typical pathologic effects were recorded in kidney and liver tissue sections. CONCLUSIONS: Intraperitoneal injection of Safflower extract at dosages of 50-200 mg/kgbw led to damage in the liver and kidney tissues, so that the concentration of 200 mg / L had severe histological complications in these tissues. Hence some limitations must be taken into account for using this extract as immune-stimulant or vaccine adjuvant.