Female nude mice were infused with 5.0, 3.0, or 1.0 μg/day of synthetic human parathyroid hormone-related protein (PTHrP) or control diluent with subcutaneous Alzet miniosmotic pumps for 7 days. Serum calcium was increased (p < 0.01) on days 3 (13.9 mg/dl), 5 (13.6 mg/dl), and 7 (12.9 mg/dl) in mice infused with PTHrP at 5.0 μg/day compared with control nude mice (8.8 mg/dl). Serum calcium was significantly increased to a lesser degree in mice infused with 1.0 μg/day PTHrP (day 3) or 3.0 μg/day (days 3 and 7). Serum phosphorus was decreased (p < 0.01) in all three groups of mice infused with PTHrP (4.6 mg/dl, 5.0 μg/day; 6.7 mg/dl, 3.0 μg/day; and 6.4 mg/dl, 1.0 μg/day) compared with controls (8.5 mg/dl). Serum 1,25-dihydroxycholecalciferol was increased (2.4-fold) in mice infused with PTHrP (5.0 and 3.0 μg/day). The urinary calcium-creatinine ratio (0.74 compared with 0.034 in controls) was increased (p < 0.03) in mice infused with PTHrP (5.0 μg/day), but the urinary phosphorus-creatinine ratio was not different from that in controls. The urinary cAMP-creatinine ratio was increased (1.6-fold) in mice infused with PTHrP (5.0 μg/day). Static bone histomorphometry revealed increased (p < 0.01) trabecular bone area, osteoblast perimeter, osteoid perimeter, osteoid width, wall width, osteoclast area, number of osteoclasts, and osteoclast perimeter in trabecular bone of lumbar vertebrae from mice infused with PTHrP. Dynamic bone histomorphometry demonstrated increased (p < 0.01) double-labeled perimeter, mineralizing perimeter, and bone formation rate. The results of this study indicated that PTHrP increased serum calcium and 1,25-dihydroxy-cholecalciferol, decreased serum phosphorus, increased urinary excretion of calcium, phosphorus, and cAMP, and increased both bone resorption and formation in nude mice. PTHrP mimics the action of native PTH in vivo and is likely to be an important protein in the pathogenesis of humoral hypercalcemia of malignancy.