250 mg/kg i.p. of the methyl ester of alpha-methyl-p-tyrosine (alpha-MpT) produced a rapid increase (by 1 h) in immunoreactive plasma ACTH, whereas the hypothalamic norepinephrine (NE) concentration and plasma corticosterone (B) changed significantly only 6 h after the administration of the inhibitor. These data suggest that the norepinephrinergic inhibitory tonus of CRF-ACTH secretion depends on the availability of a rapidly utilized pool of transmitter, affected in a very short time by alpha-MpT.