Human hydatidosis is characterized by a prolonged coexistence of Echinococcus granulosus and its host without effective rejection of the parasite. This parasitic infection constitutes a major health problem in Algeria. In this study, we investigated in vivo production of nitrite (NO(2)(-) + NO(3)(-)) in sera of Algerian patients carrying different cyst locations. Nitrite (NO(2)(-) + NO(3)(-)) levels were evaluated by the Griess method. Our results indicated that the levels of nitrite were significantly higher in the sera of hydatic patients than those of healthy controls supporting the involvement of nitric oxide (NO) in antihydatic action. The levels of nitrite in sera of the patients with hepatic hydatidosis were significantly higher than those with pulmonary infection. The lower serum (NO(2)(-) + NO(3)(-)) levels were observed in the relapsing cases. In addition, (NO(2)(-) + NO(3)(-)) levels of fertile hydatic fluids were significantly higher compared to infertile fluids. Our results suggest that the presence of NO products in hydatic fluids seems to be related to the location and the fertility of hydatic cysts. The assessment of protein concentration in hydatic fluids showed that the concentration of proteins was not exclusively dependent on the fertility but on the cyst locations. The assessment of (NO(2)(-) + NO(3)(-)) production in hydatic patients may be a useful tool to evaluate effector mechanisms of NO and clinical manifestations.