In 3 separate experiments on male rats, the increase in plasma corticosterone produced by the methyl ester of a- methyl-p-tyrosine (α-MpT) was partially inhibited by L-threodihydroxyphenylserine (DOPS). The hypothalamic concentrations of norepinephrine (NE) and dopamine (DA) were reduced by α-MpT, and partial repletion of hypothalamic NE concentration was produced by DOPS without affecting the DA concentration. DOPS had a slightly greater effect when it was administered 4 h prior to sacrifice than it did when administered along with the α-MpT 9 h prior to sacrifice. The inverse relation between hypothalamic NE concentration and plasma corticosterone was shown to be curvilinear; the increase in plasma corticosterone was not marked until hypothalamic NE concentration was reduced more than 50%.