The aim of this investigation was to measure the resonance frequency of a number of implants placed in the rabbit tibia at insertion and at predetermined periods thereafter and to correlate the results with histomorphometric measurements made when the animals were sacrificed. Ten mature New Zealand White rabbits were used in the study. Two c.p. threaded titanium implants were placed in the right tibia of each animal. Resonance frequency measurements were made by screwing a small transducer onto a standard abutment mounted on each fixture. Measurements were repeated with the transducer oriented perpendicular and parallel to the long axis of the tibia for all proximal implants 14 and 28 days after placement and in 6 implants additionally at 42, 56, 93, 122 and 168 days after which all animals were sacrificed. Histomorphometric analysis comprised 2 parts; measurement of bone-implant contact area and height. A significant increase in resonance frequency was observed after 14 (405 Hz, +/- 234 Hz) and 28 (658 Hz, +/- 332 Hz) days. The increase in resonance frequency levelled after approximately 40 days and little further change was observed. The variation in bone-implant contact area was relatively small (1.8-4.9 mm2) and the range of bone-implant contact heights was also narrow (-1.5 (-)+ 1.5 mm). Values for resonance frequencies plotted against contact area and height were grouped around 10 kHz. In conclusion, it was shown that resonance frequency measurements can be made at placement and during healing in vivo and changes may be related to the increase in stiffness of an implant in the surrounding tissues.