This study was designed to investigate the effect of an extract of Nigella sativa L. (NS) on the histology and function of the thyroid glands in alloxan-induced diabetic rabbits. Fifteen New Zealand male rabbits were divided into three experimental groups: control, diabetic and NS-treated diabetic. At the start of the experiment, a single intravenous injection of 10% alloxan (150 mg/kg body weight (bwt)) was given to produce diabetes mellitus (DM) in the diabetic and NS-treated diabetic groups. After DM was confirmed in the two groups, rabbits in the NS-treated group were given oral doses of 20 ml/kg bwt of the NS extract every day for two months. Then Mood samples were taken from all 15 rabbits to measure the serum concentrations of glucose, triiodothyronine (T-3), thyroxine (T-4) and thyroid stimulating hormone (TSH). The rabbits were killed and tissue samples were taken from the thyroid glands for histopathological analysis. It was found that NS treatment decreased the elevated glucose concentrations, and raised the lowered serum triiodothyronine concentrations in diabetic rabbits. There were no significant differences between groups in the histology of the thyroid glands or in the concentrations of serum T, and serum TSH. It was suggested that oral NS treatment might decrease the diabetes-induced disturbances of thyroid hormone metabolism.