Little is known about the modifying effects of age on the skeletal response to intermittent treatment with PTH. We therefore compared the response of 63 aged (18 month old) and 61 young-adult (3 month old) C57BL/6 mice to 4 wk of daily sc injections of either vehicle or h(1-34)PTH at a dose of 95 ng/g body weight. The increase in total body bone mineral density (BMD), compared with vehicle-treated animals, was similar in aged and young-adult mice (+5.6 vs. +6.3%). Aged animals demonstrated a greater increase in spinal BMD than their younger counterparts (+12.0 vs. +5.1%, P = 0.01; absolute increment: 57 x 10(-4) vs. 28 x 10(-4) g/cm(2)). Microcomputed tomography analyses in a subset of the vertebrae showed a trend toward higher L5 trabecular bone volume fraction in the PTH-treated aged animals (+40.2 vs. +19.6%). Vertebral histomorphometry demonstrated a greater PTH-induced increase in osteoblast number in aged vs. young-adult animals (694 vs. 396 cells/mm(2)). In contrast, in the femur the PTH-induced increase in BMD tended to be greater in the young-adult than the aged animals, although this did not reach statistical significance (8.1 vs. 4.2%). The numbers of osteoblast progenitors and mineralizing colonies in cultured marrow were unaffected by PTH treatment in either group. We conclude that aging differentially impacts the regional skeletal response to PTH such that the increase in BMD in the spine is augmented, whereas that in the femur is unaffected. Effects on osteoblast progenitor recruitment do not seem to be the basis for these changes.