SUMMARY: The influence of plasma magnesium concentration on parathyroid gland function has been evaluated by perfusion at a constant rate of an isolated parathyroid gland in five goats and one sheep with whole blood of varying magnesium content. The concentration of magnesium in fresh whole blood was adjusted either by dialysis or by the addition of magnesium chloride before the perfusion. In each experiment, the concentration of calcium was maintained constant or was slightly altered to oppose the possible influence of magnesium on the rate of release of parathyroid hormone. Parathyroid hormone concentration in parathyroid venous plasma was estimated by a specific radioimmunoassay. In each experimental animal we observed that the concentration of parathyroid hormone in the effluent plasma diminished when the concentration of magnesium was raised or increased when the concentration of magnesium was lowered. These observations demonstrate a specific influence of magnesium on the rate of release of parathyroid hormone.