Previous studies have demonstrated a dynamic ingrowth of vessels into the developing callus. In this study, maturation and development of the regulation of microcirculation were followed in the callus of rabbits. In the first series, the effects of vasoactive substances on blood flow velocity, perfusion pressure, duration of effects and peripheral vascular resistance of the bone marrow in the femur and tibia were compared. In the second series, the same parameters were measured in the femur and in the developing callus 10 and 15 days following gap osteotomy of the tibia. There were no significant differences between the microcirculatory reactions of the intact femur and tibia. Basal blood flow could be verified in the callus on the 10th postoperative day. No vascular reactions could be elicited. Basal blood flow velocity was higher on the 15th day, when compared to the measurements on the 10th day. The substances elicited statistically significant differences in flow velocity, resistance and 50% recovery time in the callus on the 15th day. Blood flow reactions of the ipsilateral femoral and tibial bone marrow are identical, thus the femur can serve as a reference site for blood flow measurements in the callus. Regulation and maturation of callus microcirculation develop rapidly between the 10th and 15th days.