The 3,5,3'-triiodothyronine (T3) binding activity of solubilized nuclear proteins from rat liver was decreased when molybdate (10 mM) was present in the incubation medium in the absence of thiol reagents. The equilibrium affinity constant was reduced by 40%. The rate of degradation of T3-receptor complexes at 37 degrees C remained unchanged, but when the extracts were further reincubated in the presence of beta-mercaptoethanol, molybdate had a protective effect after 5 h incubation at 37 degrees C. In contrast, the thyroxine (T4) binding activity was not affected by heating at 37 degrees C or by molybdate. Ion-exchange chromatography confirmed the existence of a molybdate-receptor interaction: the T3-receptor complexes shifted from elution at 0.22 to 0.20 M NaCl with the progressive appearance of a small leader peak, whereas the T4-receptor complexes eluted in a large and split peak (0.22-0.4 M NaCl). The destabilizing effect on T3 binding induced by exogenous dephosphorylation is more efficiently reversed by beta-mercaptoethanol when the extracts were pretreated by molybdate. In controls, the loss of saturable T3 binding activity was recovered by 50% at a 10 mM concentration of beta-mercaptoethanol, but in the presence of molybdate, the loss of T3 binding activity was recovered by 50% at a 5 mM concentration of beta-mercaptoethanol. This molybdate-receptor interaction is similar to that with nuclear receptor models in term of (i) stabilization of hormone binding, (ii) dependency on a thiol, and (iii) reversibility of the destabilizing effect by exogenous dephosphorylation.