1. The effects of acute hypermagnesaemia on intracellular free calcium and adrenergic activity were investigated in six normotensive volunteers given intravenous magnesium sulphate for 3 h.
2. The free calcium concentration in platelets decreased after the first hour of infusion (P < 0.05), but did not remain significantly depressed after 2 and 3 h of continued infusion.
3. Plasma noradrenaline increased during the infusion (P < 0.05), with no change in plasma adrenaline.
4. The results demonstrate that the effects of intravenous magnesium sulphate on free intracellular calcium and plasma catecholamines are similar to those described with calcium antagonists.