Nowadays antibiotics are widely used in human and animal practice. As related to the use of antibiotics in patients with thyroid disease, it has been regarded that thyroid function may be altered. This is essential to proper determination of the effect of antibiotics which could altered the activity of the thyroid gland. The author had a study using penicillin and tetracycline in rabbits. 20 rabbits were divided into 2 groups of 10 animals each and were fed antibiotics for 7 days via oral route as follows; I) 150 thousands u/kg of penicillin. ii) 250mg/kg of tetracycline. On the eighth day 20uCi of radioactive Iodine 131 was administered via oral. Thus the following results were obtained. 1. Tetracycline and penicillin had no significant effect for thyroid uptake of radioactive Iodine 131. 2. Both drugs, slight but definitely, influenced upon PBI conversion ratio with elevating tendency. 3. Tetracycline and penicillin did not create any appreciable change in T-3 uptake.