Plasma levels of ionized calcium were measured before and after intravenous infusion of magnesium in two patients with tetany and hypomagnesaemia. The tetany reported in association with hypomagnesaemia seems to be a consequence of low plasma ionized calcium. The nature of the defect is uncertain, but evidence is consistent with inhibition of release of ionized calcium from the exchangeable bone pool as a result of magnesium deficiency. We thank Dr. J. A. Owen, Director of Biochemistry, and his staff for invaluable advice and assistance, and Dr. T. E. Lowe and Dr. D. Coventry for permission to investigate the second patient. Plasma total and ionized calcium levels were measured by one of us (W. G. N.).