Relatively large doses of alpha methyl paratyrosine ( α-MPT) (400 mg/kg, i.p.) to inhibit catecholamine synthesis were capable of blocking both the pre-ovulatory elevation in plasma LH and ovulation in proestrous rats. Similarly, large doses of diethyldithio-carbamate (DDC) to deplete brain norepinephrine stores also inhibited ovulation and LH release. The blockade induced by α-MPT could not be reversed by the administration of L-dihydroxyphenylalanine (L-dopa) or dihydroxyphenylserine (DOPS). Similarly, neither L-dopa nor DOPS was able to alter the blockade induced by DDC. An injection of progesterone (2 mg/rat, s.c.) increased the plasma LH values on the afternoon of proestrus and the stimulatory effect of progesterone was blocked by the administration of α-MPT prior to progesterone. Although L-dopa failed to reverse the blockade of α-MPT, a partial reversal was obtained by the administration of DOPS. DDC also blocked the progesterone-induced LH release and there was a slight reversal in animals given DOPS together with DDC. Except for the difficulties in reversing the blockade with drugs to augment norepinephrine synthesis, the results are consistent with the hypothesis that norepinephrine is involved in the pre-ovulatory discharge of gonadotropins.