The histomorphometric, material, and structural properties of the reparative tissue and bone that formed in a two-millimeter tibial defect in dogs were determined at two, four, eight, and twelve weeks after the operation. At two weeks, the tibial defect was filled mainly with undifferentiated connective tissue. After two weeks, the relative proportion of undifferentiated connective tissue decreased and the amount of bone progressively increased throughout the twelve weeks. New bone formed primarily by intramembranous ossification, with a small degree of endochondral ossification. At twelve weeks, bone occupied 62 per cent of the defect. The calcium content of the reparative tissue increased between four and eight weeks and then, at twelve weeks, reached a plateau of 77 per cent that of normal cortical bone. Anisotropy of the new bone that formed in the defect increased from 13.8 per cent to 26 per cent, and the mean width of the trabecular bone increased 27 per cent between the eighth and twelfth weeks. Maximum torque and torsional stiffness increased between two and eight weeks and then, by twelve weeks, the values leveled off at 44 and 29 per cent of the values for intact bone. Indentation stiffness increased between the fourth and twelfth weeks; at twelve weeks, it was 22 per cent of the stiffness of normal cortical bone. Indentation stiffness increased 283 per cent between eight and twelve weeks, despite insignificant changes in calcium content, amount of new bone, non-osseous space, water content, or volume of trabecular bone during this time. This change in indentation stiffness did correlate with increases in anisotropy of the new bone between eight and twelve weeks.