Available evidence indicates that serum magnesium (Mg++) levels influence the secretion rate of parathyroid hormone (PTH). Whether serum Mg++ concentrations also modify the action of PTH on its target organs has not been definitively established. The present experiments were designed to study this possibility. The effect of infusing PTH on the urinary excretion of cyclic AMP (cAMP) and PO4= was examined in 5 normal dogs at 2 different levels of serum Mg++. At normal serum Mg++ concentrations (1.89 ± 0.14 mg/100 ml), PTH infusion increased cAMP excretion from 1.76 ± 0.27 to 4.87 ± 1.00 nmol/min and fractional PO4= excretion (FEpo4) from 1.58 ± 0.36% to 23.1 ± 2.17%. When an identical amount of PTH was given to the same dogs at a serum Mg++ of 4.36 ± 0.20 mg/100 ml, FEpo4 increased to only 6.02 ± 1.89% and cAMP from 1.31 ± 0.23 to 1.89 ± 0.39 nmol/min. Identical results were obtained in thyroparathyroidectomized hypermagnesemic dogs. Increased serum Mg++ levels had no effect on the phosphaturia produced by the infusion of dibutyryl cAMP to thyroparathyroidectomized dogs. In vitro studies using rat renal cortical slices revealed a progressive decrease in cAMP production in response to PTH as the Mg++ concentrations were increased in the incubation medium. The overall results indicate that hypermagnesemia inhibits the phosphaturic response to PTH by decreasing the renal production of cAMP. Plasma magnesium, therefore, may participate in a double feedback mechanism, not only controlling the release of PTH, but also altering the biological activity of the hormone at the level of the target organ.