1. Magnesium depletion was induced through dietary magnesium deprivation in intact, thyroidectomized and thyroparathyroidectomized (T-PTx) rhesus monkeys.
2. Serum Mg2+ fell to 33% (0.22±0.01 min) of control values and serum Ca2+ decreased to 79% (1.86±0.03 mM) of control values in Mg2+-depleted monkeys. Serum Mg2+ and Ca2+ concentrations were highly correlated (r = 0.65, P < 0.001).
3. Thyroidectomy did not prevent the hypocalcaemia. T-PTx diminished but did not abolish the decrease in serum Ca2+ after Mg2+ depletion. Exogenous parathyroid hormone (PTH) increased the serum Ca2+ during Mg2+ depletion to the same extent as during control periods.
4. Serum Mg2+ was unaffected by T-PTx during control as well as depletion periods. PTH did not substantially affect serum Mg2+ in intact or T-PTx monkeys during control or Mg2+-depletion studies.
5. It is concluded that Mg2+ depletion similarly affects monkeys and humans. PTH exerts little influence on magnesium homeostasis. A state of ‘PTH resistance’ could not be demonstrated.