The binding of rat liver glucocorticoid.receptor complexes to DNA-cellulose and nuclei has been studied after activation of the complexes by heating. Subsequent exposure to pyridoxal 5'-phosphate or pyridoxal markedly inhibited this binding. In one system 0.75 mM pyridoxal 5'-phosphate or 6.5 mM pyridoxal gave 50% inhibition. Pyridoxamine 5'-phosphate, pyridoxamine, and pyridoxine did not inhibit significantly. The inhibition by pyridoxal 5'-phosphate is competitive with respect to DNA suggesting that its effect is directly on the DNA binding site of the activated receptor. The inhibition of DNA-cellulose binding by pyridoxal 5'-phosphate can be reversed by treatment with dithiothreitol or by gel filtration, but not if the modified receptor is first reduced using sodium borohydride. These results suggest that pyridoxal 5'-phosphate acts by forming a Schiff base of an epsilon-NH2 of a lysine which may be 1 residue appearing on the surface of the steroid.receptor complex upon activation. However, since pretreatment of the DNA-cellulose with the intercalating drug ethidium bormide also inhibits activated receptor binding, we conclude that the binding of the receptor involves more than electrostatic interactions between receptor positive charges and DNA phosphate groups.