The effects of 30 days treatment with chlorpropamide (5 mg/kg/day), glipizide (2.5 mg/kg/day), arnd metformnin (350 mg/kg/day), on blood glucose level, serum fructosamine, glycosylated haemoglobin (HBA₁), triiodothyronine (T₃ ) and thyroxine (T₄) in diabetic rats were investigated. Animals were randomly assigned into three equal groups, each group received a single daily dose of one of the previously mentioned antidiabetic drugs. The parameters of interest are demonstrated before, ten and thirty days after drugs' administration. In the present study, all the used drugs significantly reduced the blood glucose level after 10 and 30 days. Both fructosamine and HBA₁ are significantly elevated after 30 days of chlorpropamide and glipizide administration, while the two paramaters were non-significantly changed in the group received metformin. Triiodothyronin was significantly decreased andtu was increased in all groups after 30 days of treatment. It could be concluded that, both chlorpropamide, glipizide and metformin decreased the elevated blood glucose level in diabetic rats. The change in fructosamine and glycolated haemoglobin was not indicative to the change in blood glucose level induced by glipizide, chlorpropamaide and metformin as the 30 days of study did not cover the required period needed for such correlation to occur. Triiodothyronine and thyroxin levels were decreased and increased respectively in diabetic rats treated by chlorpropamaide or glipizide indicating a modulatory action of such antidiabetic agents on the thyroid function. Metformin lowered T₃ but showed a limited effect on T₄ level.