Although there are a number of references to the clinical aspects of fractures of the jaw (Thoma, 19421; Blair and Ivy, 19362; Padgett, 19383; Major, 19434; Fry and co-workers, 19435a; Erich and Austin5b) relatively few experimental studies have been reported. Consequently, in view of present injuries caused by war, a roentgenographic and histologic study of the reactions of the jaw to fracture is timely. Experimental fractures of the mandible in the rat in particular permit a study of the reactions in the growing tooth as well as in the growing bone. While fractures of the jaw in dogs or guinea pigs have been studied (Schafer, 19236; Greve, 19277 and Grimson, 19378), no report on the effects of fractures of the jaw in the rat and on the simultaneous reaction of all the calcified structures present at the site of fracture was