Glucocorticoid -receptor complexes from intact rat thymus cells incubated with [3H]dexamethasone at 0 degree C are in the nonactivated form and do not bind to DNA-cellulose. Upon being warmed, they are transformed to activated complexes that bind to DNA-cellulose at 0 degree C. We have found that treatment of dexamethasone-receptor complexes with the sulfhydryl-modifying reagents methyl methanethiosulfonate ( MMTS ) and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), either before or after the warming, inhibits subsequent binding to DNA-cellulose. The effects of these reagents can be reversed at 0 degree C by dithioerythritol and other sulfhydryl-containing compounds. These results provide the first clear evidence that sulfhydryl-modifying reagents inhibit the binding of activated dexamethasone-receptor complexes to DNA-cellulose and suggest that sulfhydryl groups may be located in or near the DNA binding domain of the rat thymus glucocorticoid-receptor complex. Furthermore, addition of dithioerythritol at 0 degree C to nonactivated receptor complexes that have been treated with MMTS or DTNB produces a substantial increase in the capacity of these complexes to bind to DNA-cellulose, raising the possibility that sulfhydryl groups may be associated with a region on the receptor that plays a critical role in the activation process.