Magnesium (Mg) deficiency sometimes causes hypocalcemia with impaired PTH secretion although the precise mechanism remains unclear. We examined the PTH secretion in response to physiological hypocalcemic stimuli in a patient with hypomagnesemic hypocalcemia. We adopted sodium bicarbonate infusion test, which we recently developed, to evaluate the PTH response to acute decrease in plasma ionized Ca. The results showed that, before Mg replacement and when the patient was mildly hypocalcemic, absolutely no PTH release to hypocalcemic stimuli was observed. In contrast, the plasma Ca was promptly normalized following the start of Mg replacement, and brisk PTH response to hypocalcemic stimuli was obtained during the same test carried out a week after the Mg replacement. The data in this case thus suggest that: a) the acute regulation of PTH release by plasma ionized Ca is lost in the patient with hypomagnesemic hypocalcemia, and b) Mg deficiency itself is likely to be a primary cause of this disorder because the hormone response was clearly restored after shortterm Mg replacement alone.