In China, the ethylacetate extract of the herb Tripterygium wilfordii Hook f (TWEE), containing the major active ingredient triptolide, is often used with favorable effect on rheumatoid arthritis patients, in alternation with the use of prednisone. The mechanism of this therapeutic effect, however, has not been completely delineated. In this study, we studied how TWEE and prednisone affect the pituitary and adrenal glands in rats. Thirty normal male Sprague-Dawley rats (ten per group) were randomly assigned to receive: (1) TWEE (25 mg/kg, twice a day), (2) prednisone (2 mg/kg, twice a day), or (3) vehicle (control) (0.5% sodium carboxymethyl cellulose 1 ml, twice a day), orally for 30 days. Pituitary and trunk blood were collected on day 31. Adrenocorticotropic hormone (ACTH) expression in the pituitary gland was assessed morphologically by immunohistochemical techniques. Plasma ACTH concentrations and serum corticosterone concentrations were quantitatively measured by radioimmunoassay. We found that TWEE significantly increased plasma ACTH concentration and serum corticosterone concentration and dramatically increased the number of ACTH-positive cells in the pituitary. Our findings indicate that TWEE can promote the synthesis and secretion of ACTH cells--in the pars distalis of the rat pituitary gland and the production of corticosterone in the zone fasciculata of the adrenal cortex. Our results indicate that TWEE has a cortical hormone-like function and can promote adrenal cortex function by activating the hypothalamus-pituitary-adrenal axis.