To determine whether alterations in mineral handling induced by acute GH infusion were associated with changes n i vitamin D metabolism or parathyroid function, we studied 11 GH-deficient patients. Basal serum calcium, phosphorus, glucose, insulin, and somatomedin were measured in the fasting state, as were the vitamin D metabolites 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and 24,25-dihydroxyvitamin D and immunoreactive parathyroid hormone. Basal urinary calcium and phosphorus excretion and the calciuric response to an oral calcium load also were determined. These studies were repeated 4 weeks later during an overnight GH infusion (8 μg/kg-h), which raised mean (and SEM) serum GH levels to 46 ± 7 ng/ml. During GH infusion there were no changes in serum calcium or phosphorus, but plasma glucose (from 81 ± 3 to 87 ± 3 mg/dl), insulin (from 13 ± 1.7 to 25 ± 2.6 μU/ml), and somatomedin (from 0.63 ± 0.15 to 1.1 ± 0.3 U/ml) levels rose significantly relative to preinfusion values. Basal and postload urinary calcium excretion rose, but the increases were not significant. In contrast, GH had a significant antiphosphaturic effect, the renal phosphorus reabsorption threshold rising from 4.17 ± 0.17 before the infusion to 4.94 ± 0.13 mg/dl after the infusion (P < 0.002). Serum immunoreactive parathyroid hormone rose slightly from 342 ± 104 to 440 ± 78 pg/dl, but there were virtually no changes i n 1,25-dihydroxyvitamin D, 24,25-dihydroxyvitamin D, or 25-hydroxyvitamin D levels. We conclude that overnight GH infusions sufficient to raise plasma GH levels above the physiological range can modify renal handling of minerals without affecting parathyroid function or vitamin D metabolism. While the way in which GH exerts these effects remains unknown, it is of interest that in our subjects, the renal phosphorus reabsorption threshold (but not urinary calcium excretion) correlated significantly with somatomedin (r = 0.61; P < 0.01) and insulin (r = 0.048; P < 0.05) concentrations, suggesting that the antiphosphaturia after GH administration might be effected through one or more of these related peptides.