Nowadays Moringa oleifera (M. oleifera), family Moringaceae, is considered an important food source for human in many parts of the world, and as antiviral, antibacterial, sex reversal in tilapia fish and immunostimulant agent in fishes. This
study was conducted to investigate the effect of acute and chronic exposure of Moringa oleifera seeds extract on some hematological, biochemical variables and histopathological changes of tilapia fish species Oreochromis niloticus. The 96 h LC50 value of M. oleifera seeds extract to the studied fish was 138 mg/L. For acute study,the same concentration of 96 h LC50 (138 mg/L) was used while for chronic exposure,1/10 of 96 h LC50 value (13.8 mg/L) was taken. At the end of acute and chronic exposure, hematological parameters like red blood cell count (RBCs), hemoglobin(Hb), hematocrit (Hct), and mean corpuscular hemoglobin concentration (MCHC)  were significantly (P<0.05) decreased in fish exposed to seeds extract. However a significant (P<0.05) increase in white blood cell count (WBCs), mean corpuscular
volume (MCV) and mean corpuscular hemoglobin (MCH) values were observed in the exposed fish during above treatment period when compared to that of the control groups. Biochemical parameters such as glucose showed a significant decrease in acute exposure while increased significantly in chronic exposure (P<0.05). The obtained results showed a significant decrease in total protein and plasma cholesterol levels in fish exposed to acute and chronic concentrations of seeds extract while a significant (P<0.05) increase in aspartate aminotransferase (AST), alanine
aminotransferase (ALT), creatinine, uric acid and alkaline phosphatase (ALP) activity was observed. Chemical muscle composition in fishes exposed to chronic exposure showed a significant increase in muscle water content and ash content while showing a significant decrease in muscle total lipids and total protein. The studied growth
indices showed a significant decrease in body weight gain (BWG), specific growth rate (SGR) and hepatosomatic index (HSI). Histopathlogical sections showed pathological changing in gills, liver, kidney and spleen and the obtained results were discussed. This study could provide valuable information about the toxicity of M.oleifera seeds extract to Oreochromis niloticus. especially with the wide spread of using in the field of aquaculture and to establish safer limit in water purification.