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1. In ovariectomized rats pre-treated with oestrogen and progesterone, the intensity of the catecholamine fluorescence was determined by micro-fluorimetry in the cell bodies of the tubero-infundibular dopamine neurones (situated in the arcuate and periventricular hypothalamic nuclei). The concentrations of follicle-stimulating (FSH) and luteinizing hormone (LH) in serum and pituitary were estimated by bio-assay.2. The characteristic changes in fluorescence intensity of the tuberal catecholamine-containing nerve cell group that are induced by unilateral intermittent electrical stimulation of the arcuate nucleus or of the medial preoptic area during 5-60 min, were accompanied by a marked rise in serum LH concentration. With arcuate stimulation, the rise in serum LH paralleled the increase in the fluorescence intensity of the nerve cell bodies, whereas with preoptic stimulation, the initial increase in cellular fluorescence intensity preceded the rise in serum LH. Preoptic stimulation also caused a moderate fall in pituitary LH which was simultaneous with the increase in fluorescence intensity. No systematic changes in FSH concentration were observed during the time interval studied.3. Pre-treatment with the tyrosine hydroxylase inhibitor alpha-methyltyrosine completely blocked the increase in fluorescence intensity caused by electrical stimulation of the arcuate nucleus but did not prevent the rise in serum LH.4. It is concluded that the tubero-infundibular dopamine neurones serve as an intermediary between the medial preoptic area (and possibly other forebrain structures) and neurones containing releasing factors. Under the present experimental conditions, stimulation of the tuberoinfundibular dopamine neurones increased the release of LH, probably by promoting the release of LH-releasing factor.