In the present study an attempt was made to investigate the acute toxicity of atrazine (ATR) a herbicide on an economically important fish, Cyprinus carpio. In which, 24 h median lethal concentration (24 h LC₅₀) of herbicide, atrazine to C. carpio was 18.5 ppm.  Due to fish exposure to 24 h LC₅₀ of atrazine for 24 h, red blood cells (RBCs) count (-63.17%), hemoglobin (-27.35%), plasma glucose (-6.78%) and plasma protein (-18.73%) levels were decreased; whereas white blood cells (WBCs) count (+3.73%) was enhanced. The differences in haematological and biochemical values were statistically significant (p < 0.05). However, WBC count was not significantly changed. The alterations of the above parameters could be used as an important tool for the assessment of pathological conditions of fish.

Key words: Atrazine, acute toxicity, hematology, biochemical, Cyprinus carpio.