The relative rate of ovalbumin transcription was significantly increased (P less than 0.001) when purified chick liver glucocorticosteroid receptor (GR) was incubated with purified nuclei prepared from the oviducts of diethylstilboestrol (DES)-primed chickens 24 h after oestrogen withdrawal. This increase was observed whether GR was bound by the agonist triamcinolone acetonide (TA, +80.3%) or the antiglucocorticosteroid RU 486 (+89.4%). No significant increase (P greater than 0.05) in the relative rate of ovalbumin transcription occurred when oviduct nuclei were incubated with TA or RU 486 alone or when purified GR was incubated with chicken liver nuclei prepared from the same animals. However, glycerol gradient studies demonstrated that the sedimentation coefficient of purified TA- and RU 486-GR complexes was shifted from 8.5S to 4.4S upon incubation at 25 degrees C for 30 min with purified nuclei. Furthermore, the binding of in vitro transformed (4S) TA- and RU 486-GR complexes to either DNA-cellulose or mouse mammary tumor virus (MMTV) long terminal repeat (LTR) DNA were indistinguishable when performed under steady-state conditions. These data showing an agonist behaviour of the transformed 4S-form of RU 486-GR complexes, together with those previously reported, suggest that in vivo the antagonistic activity of RU 486 stands at the level of receptor transformation.