The effects of sodium molybdate on the transformation of mouse uterine cytoplasmic estradiol . receptor complex has been studied. If molybdate is added to cytosol containing the nontransformed estradiol . receptor complex, it blocks the receptor transformation which otherwise occurs during incubation at 28 degrees C. The inhibition by molybdate is apparent both in sucrose gradient centrifugation analyses of estradiol . receptor complexes and [3H]estradiol dissociation kinetic studies. A molybdate-sensitive change of the nontransformed estradiol . receptor complex can also be inferred from behavior of the estradiol . receptor complex on low ionic strength sucrose gradients.