We have reported that dietary magnesium (Mg) deficiency induced both histaminaemia and hypomagnesaemia in rats. In this study, the effects of dietary Mg deficiency on the reactivity of the isolated thoracic aorta to some vaso-active amines were studied ex vivo. Adult male Wistar rats were fed on a Mg-deficient diet (Mg 0.001%) and a control diet (Mg 0.07%) for 30 d. Sensitivity to KCl and adrenaline showed no significant change during Mg deficiency. In Mg-deficient rats, the sensitivity to noradrenaline was higher than in the control group. Removal of calcium ions from the medium in the tissue bath resulted in a greater decay of noradrenaline-induced contraction in Mg-deficient rats than in the control rats. Endothelium-dependent relaxation was also examined in isolated aortic strips precontracted with noradrenaline. In rats which were fed with the Mg-deficient diet for 15 days, histamine produced less relaxation than in the control group, but acetylcholine produced the same degree of relaxation as in the controls. These results showed that hypomagnesaemia and histaminaemia might influence the reactivity of the aorta in vivo.