Incubation of hamster uterine cytosol with millimolar concentrations of sodium molybdate prior to addition of labeled steroid increased recovery of progesterone receptor 2-fold. This stabilizing effect of molybdate was also manifest on gel electrophoresis of the receptor. In the absence of molybdate, no specific [3H]progesterone binding was detectable on polyacrylamide gel electrophoresis. But, in the presence of 5 mM sodium molybdate, a [3H]progesterone-binding species was clearly evident on the gels. The radioactivity associated with this binding was displaceable by unlabeled progesterone but not by cortisol and depended on the concentration of [3H]progesterone employed, suggesting that this binding species is a progesterone receptor. Molybdate treatment produced a small increase in receptor size on low-salt sucrose gradients (from 6-7 to S to 7.5 S). There was no effect of molybdate of receptor sedimentation in the presence of high salt (0.3 M KCl). Further analysis of this phenomenon by gel filtration suggested that this molybdate-mediated increase in receptor size was due to receptor aggregation. In low-salt buffers, molybdate treatment markedly increased the proportion of receptors contained in large aggregates (Stokes radius greater than 8.0 nm). Again, this effect was abolished in the presence of high salt. In conjunction with receptor stabilization, molybdate prevented binding of uterine progesterone receptor to DNA--cellulose. These findings suggest that sodium molybdate stabilizes the unliganded, unactivated form of the receptor. Moreover, these effects seem to be mediated through a direct interaction of molybdate with the receptor, one which results in receptor aggregation.