The haemodynamic effects of a single 3 g dose of magnesium sulfate administered by slow (1 min) intravenous injection were evaluated in 16 patients with coronary disease about to be explored by coronary arteriography. The haemodynamic effects were transient, with maximal values at the end of the injection and return to baseline values within less than 3 min. They consisted of peripheral vasodilatation with a decrease in systolic aortic pressure (128 +/- 18 mmHg versus 113 +/- 17 mmHg, p less than 0.05), an increase of cardiac index (3.0 +/- 0.4 versus 3.8 +/- 0.06 1/min/m2, p less than 0.001) and a fall in peripheral arterial resistance (1168 +/- 203 versus 919 +/- 29 dyn/s/cm-5, p less than 0.01). This action was accompanied by a moderate increase in contractility (Vmax) (1.63 +/- 0.34 versus 1.87 +/- 0.47 CIR/s, p less than 0.01) without changes in the relaxation index T (37 +/- 8 versus 67 +/- 9 s-1, NS), but with concomitant increase in heart rate (80 +/- 12 versus 67 +/- 10 beats/min, p less than 1.10(-4]. It is concluded that the haemodynamic effects of magnesium sulfate are moderate and transient and that this substance can be used safely as antiarrhythmic agent, even in case of marked deterioration of the left ventricular function.