RU40555 is a recently available glucocorticoid receptor (GR) antagonist that differs from RU486 by a methyl radical. We have used the mouse fibroblast cell line L929 to study the in vitro effects of RU40555 on GR translocation and function and on the membrane steroid hormones transporter. The results showed that: 1) RU40555 competed for the binding of labelled dexamethasone (Dex) with a K(i) of 2.4 nM; 2) both RU40555 and RU486 were equally potent inhibitors of Dex-induced GR-mediated gene transcription; 3) maximum GR translocation induced by micromolar concentrations of Dex and the GR antagonists was approximately 30-55% loss in the cytoplasmic GR and approximately 40-90% increase in the nuclear GR (assessed by GR immunostaining in cytoplasm and nucleus and western blots of immunoprecipitated GR protein in cytosolic and nuclear fractions) and was similar for the two antagonists; 4) at nanomolar concentrations, RU40555 and RU486 induced more GR translocation than Dex (assessed by [(3)H]Dex binding and western blot of immunoreactive GR in the same cytosolic homogenates); 5) blocking the steroids membrane transporter with verapamil (100 microM) in the presence of Dex (10 nM) increased GR translocation to levels similar to those induced by RU40555 (10 nM) and RU486 (10 nM) alone; 6) verapamil did not affect GR translocation in the presence of RU40555 or RU486. These data demonstrate similar quantitative effects on GR translocation by RU486 and the new GR antagonist, RU40555. Moreover, RU40555, like RU486, is an effective GR antagonist. Finally, there is no evidence that the intracellular concentrations of RU40555 or RU486 are regulated by the steroids membrane transporter in L929 cells.