The glucocorticoid receptor antagonist mifepristone has been shown to rapidly and effectively ameliorate symptoms of psychotic major depression. To better understand its mechanism, we investigated mifepristone's cellular effects, and found that it rapidly reversed a chronic corticosterone-induced reduction of adult neurogenesis in rats. Unlike other antidepressants, mifepristone is particularly potent in a high corticosterone environment. These data indicate that similarly to its clinical efficacy, mifepristone's effects on adult neurogenesis are rapid and positive, and may therefore be important for its mechanism of action.