Parathyroid extract (PTE) had neither calcemic nor phosphaturic effects in 8 alcoholic patients who had both hypocalcemia and hypomagnesemia. Correction of the hypomagnesemia by intravenous infusion of magnesium sulfate for 3 days resulted in disappearance of PTE refractoriness and correction of serum calcium. Two additional hypomagnesemic alcoholics remained refractory to PTE during treatment with multivitamin supplements including large amounts of vitamin D, but they responded briskly to PTE following correction of hypomagnesemia. One non-alcoholic patient who had steatorrhea, hypocalcemia and hypomagnesemia was also resistant to PTE until serum magnesium had been normalized. These findings indicate that the calcemic and phosphaturic effects of parathyroid hormone are magnesium dependent, and that hypocalcemia, which occurs in severely hypomagnesemic states, is due to parathyroid hormone unresponsiveness.