Leptin, the ob gene product, secreted by adipocytes, controls overall energy balance. We investigated leptin effects on bone metabolism using male leptin-deficient obese (ob/ob), which had lower bone mineral density (BMD) and shorter femurs than lean (?/+) controls. Serum concentrations of calcium, phosphate, tartrate-resistant acid phosphatase (a bone resorption marker) and alkaline phosphatase, and urinary calcium and phosphate excretion were significantly elevated in ob/ob mice, while urinary concentrations of deoxypyridinoline did not differed between ob/ob and control mice. Since ob/ob mice develop severe hypogonadism, testosterone was administered to these mice for 10 weeks (5 mg/kg, sc, twice weekly); this did not affect femoral BMD. Control and ob/ob mice showed similar vitamin D receptor densities in bone and kidney; the obese mice had marked increase in serum 1,25-dihydroxyvitamin D₃ [1,25-(OH)₂D₃] and in mRNA expression and activities of renal 25-hydroxyvitamin D₃-1-hydroxylase (CYP27B1) and -24-hydroxylase (CYP24) than control mice. Leptin administration to ob/ob mice (4 mg/kg body weight, i.p., every 12 h for 2 days) greatly reduced mRNAs and activities of 1-hydroxylase and 24-hydroxylase. Elevated concentrations of serum calcium, phosphate and 1,25-(OH)₂D₃ were normalized by its treatment. Thus, leptin suppresses renal gene overexpression for 1-hydroxylase and 24-hydroxylase, and corrects increased serum concentrations of calcium and phosphate in ob/ob mice. Low BMD in leptin-deficient mice thus appears related to stimulation of bone resorption by 1,25-(OH)₂D₃.