Parathyroid function was studied in two infant sisters with primary hypomagnesemia while they were both hypomagnesemic and hypocalcemic. In one of the infants, plasma immunoreactive parathyroid hormone (iPTH) was elevated, the calcemic response to exogenous parathyroid hormone (PTH) was absent, and the phosphaturic response was normal. Restoration of serum magnesium with i.v. magnesium corrected the hypocalcemia, with no further rise of plasma iPTH. In the other infant, plasma iPTH was undetectable, and exogenous PTH produced both phosphaturic and calcemic responses. Normalization of serum magnesium with i.v. magnesium resulted in a prompt release of endogenous PTH and correction of the hypocalcemia. These findings suggest that, in the first patient, hypocalcemia was associated with lack of response of the bone to both endogenous and exogenous PTH, while in the second patient, hypocalcemia was associated with inhibition of PTH release and a normal calcemic response to exogenous PTH. The factors that determine whether magnesium deficiency will result in inhibition of PTH release, in a lack of response of the bone to endogenous and exogenous PTH, or both, remain to be clarified.