The effects on hypothalamic-pituitary-adrenal function and on negative feed-back action of a synthetic adrenal steroid, prednisolone acetate, have been studied in female Wistar rats treated with alpha-methyl-p-tyrosine (alpha MpT) and L-dihydroxyphenylalanine (L-DOPA) administered for six consecutive days. The parameter for study was taken from the plasmatic changes of corticosterone levels. alpha-MpT injection determines an increase of plasmatic corticosterone values, while L-DOPA injection produces the opposite effect, decreasing corticosterone levels as compared to the saline control group. The simultaneous administration of prednisolone acetate 250 micrograms/kg weight s.c. for six consecutive days to alpha-MpT or L-DOPA determines the following effects: a) With alpha-MpT the negative feed-back action of prednisolone was not modified. b) With L-DOPA this action of the steroid is cancelled and the plasmatic corticosterone values are increased significantly. These changes have been related to modifications of the catecholamines content in the brain. The importance of the catecholamines on the activity of the hypothalamic-pituitary-adrenal axis is discussed under experimental condition, specially the effects of L-DOPA, which depends for its activity on the presence or absence of prednisolone.