The aim of the study was to determine the effect of PTH, the antiresorptive agents estrogen and bisphosphonate (Risedronate), and also the combination of PTH with the antiresorptive drugs on vertebral bone mass and biomechanical competence in a sexually mature, ovariectomized rat model. A total of 138 3-month-old Sprague-Dawley rats were randomized into seven groups: (1) sham operated (control); (2) ovariectomized (OVX); (3) OVX plus estrogen; (4) OVX plus bisphosphonate [Risedronate (NE)]; (5) OVX plus hPTH (1-34); (6) OVX plus hPTH (1-34) and estrogen; and, finally, (7) OVX plus hPTH (1-34) and Risedronate. Treatment regimens were initiated 4 weeks after OVX and were continued for 5 and 15 weeks for each treatment group. Changes in bone mass (ash content) and biomechanical competence were assessed on lumbar vertebral body L4. The results revealed that the Risedronate-treated OVX animals had a higher vertebral bone mass than the OVX group. hPTH (1-34) on its own had a pronounced anabolic effect and increased bone mass 20-25% and bone strength 70-80% over control levels. Neither combination therapy with estrogen nor with Risedronate provided any further advantage. The combination of PTH with Risedronate, though, seemed to allow a continued increase in both bone mass and strength during the whole treatment period.(ABSTRACT TRUNCATED AT 250 WORDS)