Immobilization stress (1 h) induced discrete reductions in noradrenaline (NA) levels in the posterior periventricular hypothalamic region and in the paraventricular hypothalamic nucleus, and a decrease in dopamine (DA) turnover in the medial palisade zone (MPZ) of the median eminence, but failed to induce regional increases of hypothalamic NA turnover. Stress also stimulated the secretion of ACTH, corticosterone and prolactin, while vasopressin, LH and FSH serum levels were unaffected. The stress induced reduction of DA turnover in MPZ may mediate the stress induced increase of prolactin secretion. Nicotine (0.3 mg/kg, s.c., 1 h) did not by itself significantly influence catecholamine (CA) turnover in the various CA nerve terminal systems analyzed in the hypothalamus, but reduced NA levels in the subependymal layer (SEL) of the median eminence. Nicotine administration did not affect the serum levels of any of the hormones evaluated. Nicotine counteracted to a minor degree the immobilization stress-induced reduction in NA levels, and also the stress-induced secretion of ACTH, but not of prolactin suggesting the involvement of noradrenergic mechanisms possibly in the paraventricular nucleus in the nicotine modulation of stress induced increases of ACTH secretion. The nicotine-induced reduction of NA levels in SEL was blocked by stress as well as the tendency for nicotine induced increases of dopamine (DA) turnover in the medial and lateral palisade zones of the median eminence indicating opposing influences of immobilization stress and nicotine on at least some hypothalamic CA systems.