Introduction: In recent years, considerable attention has been paid to the problem of nitrates due to its intensive use as agricultural fertilizers which reach to humans and animals by different routes. Purpose: To study the effects of sub-lethal dose of nitrate in a short-term experiment (60 days) on thyroid gland and the possible ameliorative effect of vitamin C. Material and Methods: Twenty adult male albino rats were divided into four groups and treated daily for 60 days as follows: Group I (control) received distilled water, Group II received 20mg∕kg vitamin C, Group III received 50mg/kg sodium nitrate (NaNO3) and Group IV received 50mg/kg NaNO3+20mg∕kg vitamin C. All supplementations were given orally by gastric tube. At end of the experiment, all animals were sacrificed; blood samples were subjected to hormonal assay (T3, T4 and TSH levels) and measurement of superoxide dismutase (SOD) and malondialdehyde (MDA). Also, thyroid glands specimens were collected for the light and electron microscopic study Results: Light microscopic examination revealed that nitrate caused disturbtion in the thyroid architecture, in the form of poorly developed follicles that were heterogeneous because of the variable size of both cells and colloidal spaces. Enlarged deformed follicles with scanty vacuolated colloid, degenerative follicles in the form of empty, fused follicles and their lumen were filled with exfoliated cells were encountered. Ultrastructurally, most of the follicular cells showed irregular nuclei, irregular dilation of rER and many vacuoles. Signs of follicular hyperactivity were manifested by the appearance of microfollicles, follicular epithelial cells stratification and an apparent increase in the number of lysosomes. Also hypetrophy and hyperplasia observed in parafollicular- C cells. These alterations were further confirmed by hormonal assay, which showed a significant decrease in serum T3 and T4 with subsequent increase in TSH. Also, the oxidative damage supported by MDA activity increased significantly whereas the SOD content decreased. Structural and ultrastructural results after treatment with vitamin C showed partial improvement. In conclusion, sodium nitrates appeared to have toxic effects in the thyroid gland and this toxicity is partially improved by treatment with vitamin C. It is recommended that the use of sodium nitrate must be limited and encourages dietary addition of vitamin C as antioxidant to prevent the toxic effects.