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In this study, the role of nitric oxide (NO) in the pathogenesis of hydatidosis and the interaction with effects of anthelmintic drugs, albendazole and praziquantel, were examined in larval infection caused by protoscolices obtained from hydatid cysts of sheep liver in Albino Balb/c mice. Animals were divided into ten groups including controls and infected groups. Larval infection was established with intraperitoneal injection of protoscolices. Eight months after infection with protoscolices, the infected animals were divided into 6 groups. The infected animals were given a selective inhibitor of inducible nitric oxide synthase (iNOS) L-N6-(1-Iminoethyl) lysine-hydrochloride (L-NIL), NO donor sodium nitroprusside (SNP), albendazole and praziquantel as anthelmintic drugs for 7 days. In addition, control groups were composed of intact group, control, anthelmintic drugs + L-NIL, and anthelmintic drugs + SNP. The liver and blood samples were taken for cytological, histological, immunohistochemical and biochemical analyses 7 days after treatments at the end of experiment. The animals injected with protoscolices showed histopathological changes including inflammation areas, infiltration and accumulation of leukocytes, dilation of sinusoids, and damage in endothelial cells and hepatocytes at light microscopy. Electron microscopy were revealed severe damage in sinusoidal endothelial cells, leukocytes especially eosinophils in sinusoid lumens and disorganization in endoplasmic reticulum and nuclear membrane. Endothelial nitric oxide synthase (eNOS) and iNOS reactions were increased in the tissue. Anthelmintic drugs decreased inflammation areas and damages; however, it did not change NOS reactions in the animals given protoscolices. L-NIL and SNP diminished both iNOS and eNOS reactions. Unlike the group administered the inhibitor, SNP treated group exhibited less inflammation areas. Combination of these substances and drugs resulted in decreased inflammation areas. eNOS and iNOS reactions decreased in the drugs and SNP administered group, while only iNOS reaction was decreased in L-NIL given infection group. In addition, the infected groups which received SNP displayed expanded sinusoids and hepatocytes with vacuoles, intriguingly. While levels of serum nitrite/nitrate elevated only in the infection group given drugs and SNP, it decreased in the L-NIL administered group. Tissue level of malondialdehyde increased in infection groups with drugs and SNP. In conclusion, the results indicated that NO plays an important role in the pathogenesis of hydatidosis.