1 There is evidence to suggest that magnesium (Mg2+) is beneficial in the treatment of a number of conditions, including pre-eclampsia and acute myocardial infarction. The mode of action of Mg?; in these conditions is not clear, although the vasodilator properties of Mg2+ are well documented both in vitro and in vivo. 2 Previously, we demonstrated that i.v. infusion of magnesium sulphate (MgSO4) alone, or in the presence of vasoconstrictors, caused increases in flow and conductance in the common carotid, internal carotid and hindquarters vascular beds, in conscious rats. Therefore, the objective of the present study was to investigate the regional and subregional changes in haemodynamics in response to the vasoconstrictor peptide endothelin-l (ET-1) and MgSO4 in more detail, using the coloured microsphere reference technique. 3 Infusion of ET-1 and MgSO4 had similar effects on heart rate and mean arterial pressure as in our previous study. Infusion of ET-I caused a rise in mean arterial pressure and a fall in heart rate, and infusion of MgSO4 returned mean arterial pressure to control levels with no effect on heart rate. 4 The responses to MgSO4 in the presence of ET-1 showed considerable regional heterogeneity with blood flow increasing (e.g. skeletal muscle), decreasing (e.g. stomach) or not changing (e.g. kidney). Of particular interest was the finding that MgSO4 caused increases in flow in the cerebral and coronary vascular beds. 5 This, and our previous studies, have shown that MgSO4 can reverse vasoconstriction in a number of vascular beds, and indicate that this compound may have therapeutic benefit in conditions associated with vasospasm.