Noradrenaline plays a key role in the initiation of ovulation in nonprimate species. A similar noradrenaline role in the primate has not been established experimentally. We utilized the ovariectomized-oestrogen-supplemented (OVX + E) rhesus macaque to examine the effects of intravenous (i.v.) infusion of oestradiol-17beta (E2) on the activity of the brain noradrenaline system. Experiment 1 established the induction of a preovulatory surge-like release of luteinizing hormone in OVX + E monkeys by i.v. infusion of E2 (OVX + E + E2). In experiment 2, a marked increase in hypothalamic microdialysate noradrenaline concentrations occurred after identical E2 infusion into the OVX + E monkeys that were used in experiment 1. In experiment 3, tyrosine hydroxylase (TH) mRNA expression in the locus coeruleus of the brainstem increased at various times after E2 infusion as determined by semiquantitative in situ hybridization. The amount of TH mRNA in OVX + E + E2 animals was higher (P < 0.05) than that in either the OVX + E or OVX monkeys; no difference was found in the latter two groups. Moreover, selected locus coeruleus sections from E2-infused monkeys were examined for the localization of oestrogen receptors (ER) by in situ hybridization. Both ER-alpha and ER-beta mRNAs were expressed in the locus coeruleus, although the expression was greater for ER-alpha than for ER-beta. We conclude that i.v. infusion of E2, which induces a preovulatory surge-like release of LH, stimulates brain noradrenaline activity; this enhanced activity likely involves an ER-mediated process and is reflected by hypothalamic noradrenaline release and locus coeruleus TH mRNA expression. The results support the concept that noradrenaline can influence the E2-stimulated ovulation in nonhuman primates and that the brainstem is one of the components in this neuroendocrine process.