The influence of intermittent parathyroid hormone (PTH(1-34)) administration on callus formation and mechanical strength of tibial fractures in rats was investigated after 20 and 40 days of healing. A dose of 60 microg of PTH(1-34)/kg/day and 200 microg of PTH(1-34)/kg/day, respectively, was administered during the entire periods of healing, and control animals with fractures were given vehicle. The dose of 200 microg of PTH(1-34)/kg/day increased the ultimate load and the external callus volume of the fractures by 75% and 99%, respectively, after 20 days of healing and by 175% and 72%, respectively, after 40 days of healing. The dose of 60 microg of PTH(1-34)/kg/day did not influence either ultimate load or external callus volume of the fractures after 20 days of healing, but the ultimate load was increased by 132% and the external callus volume was increased by 42% after 40 days of healing. During the healing period, the callus bone mineral content (BMC) increased in all groups. After 40 days of healing, the callus BMC was increased by 108% in the 200 microg of PTH(1-34)/kg/day group and by 76% in the 60 microg of PTH(1-34)/kg/day group. Both doses of PTH(1-34) steadily augmented the contralateral intact tibia BMC (20 days and 40 days: 60 microg of PTH (1-34)/kg/day 9% and 19%, respectively; 200 microg of PTH (1-34)/kg/day 12% and 27%, respectively) and bone mineral density (20 days and 40 days: 60 microg of PTH(1-34)/kg/day 11% and 12%, respectively; 200 microg of PTH(1-34)/kg/day 11% and 15%, respectively).