Young male Sprague-Dawley rats were fed 0, 1, 10, or 100 ppm of polybrominated biphenyls (PBB) in iodine-deficient, iodine-adequate (0.2 ppm), or iodine-excess (1000 ppm) diets. Six rats in each of the 12 groups were killed at 30 days and the remaining six in each group at 60 days. Growth rates were similar in all rats fed diets containing 0, 1, or 10 ppm PBB but were slower from 30 to 60 days in rats given 100 ppm PBB. Results of routine hematologic examinations and urinalyses were essentially normal. Although liver weights were substantially increased by PBB, the smallest increases were in rats fed an iodine-deficient diet. Thyroid weights were increased by iodine deficiency and by 10 and 100 ppm PBB. Electropherograms of serum proteins, serum lipoproteins, and LDH isozymes at 60 days from rats given PBB indicated hepatic alterations, but changes were least dramatic in rats fed an iodine-deficient diet plus PBB and most severe in rats fed iodine-excess diets plus PBB. Hepatic lesions were basically similar to those previously described except that bile duct proliferation was seen at 60 days only in rats fed an iodine-deficient diet and 100 ppm PBB. Histologic changes in thyroid glands were associated with iodine deficiency and with PBB. The iodine-excess diet plus 100 ppm PBB induced squamous metaplasia of respiratory bronchiolar epithelium. These results indicate interrelationships between PBB and iodine which may affect the toxicosis caused by PBB.