Obese (ob/ob) mice exhibit impaired hepatic thyroid hormone action that is mediated, at least in part, by a reduced nuclear 3,5,3'-triiodothyronine (T3) receptor occupancy. The possibility that lowered occupancy in obese mice may be caused by decreased transport of T3 across the hepatic plasma membrane was examined by measuring the unidirectional influx of [125I]T3 into livers of 8- to 10-wk-old obese and lean mice using a tissue-sampling portal vein-injection technique. Influx of [125I]thyroxine (T4), a substrate for T4 5'-deiodinase, was also measured. Unidirectional clearance of T3 and T4 was 64 and 80% lower, respectively, in obese mice than in lean mice. Hepatic T3 and T4 uptake was nonsaturable in both lean and obese mice, suggesting that transport occurs by lipid-mediated free diffusion. Clearance of another lipid-soluble hormone, hydrocortisone, was also lower in obese mice than in lean mice. Decreased membrane permeability to the above hormones in obese mice may result from reported changes in membrane lipid composition. In conclusion, decreased hepatic thyroid hormone uptake may contribute to impaired thyroid hormone action and T3 production in livers of obese mice.