Noradrenaline when injected into the third ventricle induces ovulation in rats (Rubinstein & Sawyer, 1970; Craven & McDonald, 1971) and stimulates release of luteinizing hormone (LH) (Kamberi, Mical & Porter, 1970). The compound 6-hydroxydopamine causes a long-lasting depletion of noradrenaline and dopamine (Bartholini, Richards & Pletscher, 1969; Uretsky & Iversen, 1970) and destruction of brain adrenergic terminals (Bloom, Algeri, Groppetti, Revuelta & Costa, 1969). After destruction of the nerve terminals supersensitivity in some responses to noradrenaline has been found to develop (Palmer, 1972; Kalisker, Rutledge & Perkins, 1973). In the present work it is shown that noradrenaline (NA) at doses so small as to be ineffective in normal rats, markedly increases plasma LH in 6-hydroxydopamine (6-OHDA) treated rats. This suggests a supersensitivity phenomenon. Male Holtzman rats (body wt 180–250 g) were used. They were housed in a temperature-controlled room and fed with laboratory rat chow and water ad libitum. Animals