Unfixed, undecalcified sections from closed fractures of rat metatarsals have been studied by quantitative cytochemistry from the first week after fracture to late in the fracture-healing process. Alkaline phosphatase activity appeared to be related to calcification. Glucose 6-phosphate dehydrogenase activity was associated with proliferation and with calcification. Studies made with the critical electrolyte Alcian blue technique suggested that whether the ossification process is of the endochondral or membranous type might be predetermined in the relatively undifferentiated callus.