Osteotomy of the distal radius and gradual distraction were performed on 20 sheep. Bone matrix synthesized between the osteotomized bone ends was biochemically characterized at 3, 5, 7, and 14 days after the start of distraction. The deposition of mineral was negligible during the 14-day period. No change was observed in the amount of organic matrix, which comprised 15 +/- 1.4 percent of the wet weight of the tissue. The amount of total protein of the wet weight in the matrix increased from 4.6 to 7.6 percent between 5 and 7 days of distraction. During the first 7 days, the nonprotein component decreased from 11 to 7.3 percent. The proportion of collagen of the total matrix protein increased gradually from 29 to 59 percent. In view of the patterns of the cyanogen bromide peptides of the matrix proteins, the heteropolymer type I[1(I)2(2)(I)1] collagen was consisted to be the principle fibrillar collagen synthesized in the distraction gap. The results suggest that tension-stress markedly affects the synthesis of fibrillar collagen towards the formation of mature bone matrix. In this process, a preliminary lag phase probably exists during which the capacity for protein synthesis is low while the synthesis of Type I collagen has started. In the secondary phase an augmented synthesis of Type I collagen takes place.