['Membrane-stabilizing effects may be part of glucocorticoid action during high-dose glucocorticoid therapy. The present study investigates the mode of action of dexamethasone megadoses on rat liver lysosomal membranes.', 'Following intravenous administration of dexamethasone in rats, the release of beta-glucuronidase from liver lysosomes was assessed ex vivo as a marker for lysosomal membrane integrity.', 'Dexamethasone megadoses significantly inhibited beta-glucuronidase release 10 min post-administration by 38% (3 mg/kg dexamethasone) and 33% (10 mg/kg dexamethasone) at corresponding dexamethasone liver concentrations of 3.9 x 10(-5) mol/kg and 15.1 x 10(-5) mol/kg, respectively. Comparable inhibition of beta-glucuronidase release (34% for 3 mg/kg and 38% for 10 mg/kg) was observed 24 h after administration of dexamethasone, although dexamethasone liver concentrations had already declined to 0.09 x 10(-5) mol/kg and 0.19 x 10(-5) mol/kg, respectively. A 2-h oral pretreatment of rats with the glucocorticoid receptor antagonist RU 486 (10 mg/kg) did not alter immediate (10 min) stabilization by dexamethasone (3 mg/kg). but almost completely prevented lysosomal membrane protection 24 h after dexamethasone injection.'] Do dexamethasone megadoses stabilize rat liver lysosomal membranes by non-genomic and genomic effects?