1 There is evidence to suggest that magnesium (Mg 2+ ) is bene®cial in the treatment of a number of conditions, including pre-eclampsia and acute myocardial infarction. The mode of action of Mg 2+ in these conditions is not clear, although the vasodilator properties of Mg 2+ are well documented both in vitro and in vivo. 2 Previously, we demonstrated that i.v. infusion of magnesium sulphate (MgSO 4 ) alone, or in the presence of vasoconstrictors, caused increases in¯ow 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-1 (ET-1) and MgSO 4 in more detail, using the coloured microsphere reference technique. 3 Infusion of ET-1 and MgSO 4 had similar e ects on heart rate and mean arterial pressure as in our previous study. Infusion of ET-1 caused a rise in mean arterial pressure and a fall in heart rate, and infusion of MgSO 4 returned mean arterial pressure to control levels with no e ect on heart rate. 4 The responses to MgSO 4 in the presence of ET-1 showed considerable regional heterogeneity with blood¯ow increasing (e.g. skeletal muscle), decreasing (e.g. stomach) or not changing (e.g. kidney). Of particular interest was the ®nding that MgSO 4 caused increases in¯ow in the cerebral and coronary vascular beds. 5 This, and our previous studies, have shown that MgSO 4 can reverse vasoconstriction in a number of vascular beds, and indicate that this compound may have therapeutic bene®t in conditions associated with vasospasm.