The aim of the study was to examine the effects of increasing zinc supplementation on growth, feed efficiency and thyroid function and histology in broiler chicks. Sixty new born male broiler chicks were randomly allotted into one of four treatment groups and fed for 60 d. Zinc (Zn) was added into drinking water at the levels of 0, 125, 500, and 1000 mg Zn/L. Body weight gain were significantly higher and feed efficiency were significantly lower in chicks supplemented with 125 mg Zn/L compared with chicks supplemented with 500 or 1000 mg Zn/L at the end of the experiment. Serum Zn concentration linearly increased with the increasing level of Zn intake. Serum triiodothyronine and thyroxine levels and the diameters of follicles of thyroid gland were significantly reduced with high levels (500 and 1000 mg Zn/L) of Zn intake at the end of the experiment. It was concluded that chick receiving 1000 mg Zn/L as ZnSO4 . 7H(2)O in drinking water showed signs of Zn toxicity.