Hepatic nuclear triiodo-L-thyronine (T3) receptors were studied to investigate the mechanism of thyroid hormone resistance in Richardson's ground squirrels, Spermophilus richardsoni, during the hibernation phase of the annual cycle. The cycle is divided into an active phase and a hibernation phase, the latter composed of alternating dormancy and arousal bouts. In addition to animals in these three states, a group of cold-exposed animals was also examined (those animals held at 6 degrees which showed no indications of entering hibernation). Binding of T3, to squirrel hepatic nuclei from all groups, was characterized as high affinity, Kd ranging from 111 to 267 pM, and low capacity, 50 to 314 fmol T3/mg DNA. Based on these data, other criteria examined, and models established in the literature for other species, this binding site has been tentatively identified as a T3 receptor. Receptor concentrations in nuclei from dormant and aroused squirrels were only 15-20% of the concentration in active animals. There were no differences in the affinity of the T3 receptor over the annual cycle estimated by the in vitro assay at 24-26 degrees. The reduction in nuclear T3 receptors, together with the previously reported increase in serum binding of thyroid hormone, provides an explanation for thyroid hormone resistance during the hibernation phase in S. richardsoni.