The limited capacity, high affinity binding of 35 iodothyronine analogues by rat liver nuclei has been examined in an in vitro system. The in vitro nuclear binding of all the analogues tested was highly correlated with their published thyromimetic potencies in the intact animals. Binding and biological activity are greater for 3'-mono-than 3',5'-di-substituted iodothyronines. A 4'-hydroxyl group is essential, but the 3' substituent can be several halogen or non-halogen groups for which the distal conformation is preferred. The ether linkage can be replaced equally well by a methylene or sulfur group. The presence of both 3 and 5 groups which are limited to halogens or small alkyl groups are necessary for the maintenance of significant activity. Halogen-free iodothyronines have very low, but significant activity both in vitro and in vivo. The data provide information on the structural requirements for thyroid hormone action and further support the physiological relevance of the nuclear sites.