The purpose of this study was to evaluate the effects of fenitrothion, an organophosphothionate insecticide, on selected serum hematological and biochemical parameters of carp ( Cyprinus carpio L.) fingerlings and investigate histopathological changes after acute dose of exposure. Among those, cortisol and glucose are evaluated as responses of early life stages of carp to acute stress and hematological blood parameters are evaluated to investigate the mechanism of toxicity via histopathological changes to the standard test organism. No significant histological findings were observed in carp tissues ( gills, liver, kidneys, spleen, gonads, brain, muscle, and skin) exposed to a sublethal concentration of 10 mg/L fenitrothion and controls. Hematocrit and erythrocyte counts were decreased significantly by exposure to fenitrothion when compared to the control group (25.26% +/- 0.82%, 1455 +/- 90.3 [ 103/mu L] vs. 28.83% +/- 1.60%, 1865 +/- 74.5 [10(3)/mu L], P < 0.05), respectively. Leucocyte and thrombocyte counts did not change. Increases in plasma cortisol levels were found in the experimental group compared to controls (34.2 +/- 1.9 vs. 18.0 +/- 2.2, P < 0.05) respectively, while glucose, chloride, sodium, potassium, phosphorous, and brain MDA and FOX levels did not show significant differences from those of control ( P > 0.05). As a result, we can conclude that early life stages of fish are very susceptible to the adverse effects of toxicants.