The association between prolonged exposure to water nitrate pollution and increased blood methemoglobin level in both young and adult male rats A.M. El-Wakf; H.A. Hassan; F.G. El-said and A. El-Said Zoology department, Faculty of Science, Mansoura University. El-Wakf, A.M.; Hassan, H. A.; El-said, F.G.; El- Said, A. (2009): The association between nitrite contamination of drinking water and changes in methemoglobin level other hematological parameters in young and adult male rats. J. Egypt. Soc. Toxicol., 49:91-96. Abstract The objective of this investigation was to study the effect of prolonged exposure to water nitrate pollution on blood methemoglobin (Met-Hb) level and other hematological parameters, as well as nitric oxide (NO) level (measured as indicator for nitrate toxicity) in male rats of two different stages of life, young (3 weeks-old) and adult (12 weeks-old). Rats were administered sodium nitrate in drinking water at concentrations (100 mg/L ≈ 8.7 mg/Kg, 250 mg/L≈21.7 mg/Kg and 550 mg/L ≈ 47.7 mg/Kg) daily for four months. Obtained data showed a dose dependent reduction in the values of erythrocytes, leucocytes and platelets count, as well as Hb content, Hct%, MCV, MCH, MCHC and serum total iron in all nitrate exposed rats, with the greatest decreases being occurred among the young animals. On contrary, nitrate exposure revealed a dose dependent increase in blood Met-Hb level, as well as NO production in serum and urine of all tested rats, however the younger animal groups showed more increases than the adult ones at different nitrate concentrations. This indicated that prolonged exposure to water nitrate pollution is a risk factor for developing increased Met-Hb level and the other hematological disorders in all the tested rats, particularly the young ones which may contribute to the excess production of NO in response to nitrate exposure. Key words: Nitrate pollution , methemoglobin level , nitric oxide. Introduction Drinking water pollution with high nitrate concentration is potentially harmful to human health (Ward et al., 2005; Arumi et al., 2006). Detailed investigation indicated an association between high ingestion of nitrate polluted drinking water and