The effects of interfragmentary sliding (shear) motion, axial motion, and locked external fixation on the healing of mid-tibial closed fractures were studied in fifty-six skeletally mature New Zealand White rabbits. The fractures were fixed with use of a four-pin, double-bar frame and were allowed to heal for either two or four weeks. Four experimental conditions were evaluated: transverse and oblique fractures treated with a locked external fixator (Groups 1 and 3, respectively), transverse fractures treated with an axially telescoping fixator (Group 2), and oblique fractures treated with a sliding oblique fixator (Group 4). The maximum interfragmentary motion, recorded in vivo with an electronic motion sensor that was attached to the fixator, was 0.6 millimeter in Group 2 during the first week and then declined rapidly. In contrast, the motion in Group 4 exceeded 1.5 millimeters during the first week. The circumference of the callus in Group 4 was 11 to 23 per cent greater than that in the other groups at both two and four weeks (p < or = 0.02). At two weeks, torsional stiffness, strength, and energy absorption were comparable among Groups 1, 2, and 3. The increase in healing was most rapid for Group 4; by four weeks, the torsional strength and energy to failure of the fractures in Group 4 exceeded those in the other groups (p < or = 0.025) and reached or exceeded those of intact bone. Apparently, oblique sliding (shear) motion promoted greater cartilage differentiation and expansion of the peripheral callus than did axial motion or locked external fixation.