The relationship between hypomagnesaemia and the secretion of parathyroid hormone was studied. Serum parathyroid hormone (PTH) concentrations were measured in 63 randomly selected patients and 54 hospitalized demented patients using carboxyl-terminal PTH53-84 (h-PTH53-84), mid-region PTH44-68 (m-PTH44-68) and intact PTH1-84 kits. Both groups of patients were further subdivided into those with near normal serum magnesium concentrations (1.7 - 3.2 mEq/l) and those with hypomagnesaemia (1.2 - 1.6 mEq/l). Serum h-PTH53-84, m-PTH44-68 and PTH1-84 concentrations in both the randomly selected and the demented patients with hypomagnesaemia were significantly lower than those in the groups with near normal serum magnesium concentrations. The results suggest that parathyroid hormone secretion was inhibited in randomly selected patients and dementia patients with hypomagnesaemia.