A histomorphometric study was carried out on healing defects in the ribs of beagles. A transverse fracture was made surgically in the midshaft of the left 9th and 10th ribs. Ten beagles received injections of either a buffer vehicle (n = 4) or prostaglandin E1 (PGE1) at a dose of 0.4 mg/day locally (n = 6) into the fracture sites for a 10-day period and were killed 30 days after the surgery. Double-pulsed fluorescent labels were given with each of two fluochrome markers, calcein before surgical treatment and oxytetracycline hydrochloride before killing. The objectives were to determine manifestations of the regional acceleratory phenomenon (RAP) as changes in regional remodeling in the haversian envelope induced by fracture, effects of PGE1 on modification of the RAP in the haversian envelope, and systemic effects of PGE1 on remodeling changes of the contralateral matching sites. The differences in haversian remodeling between the injured and uninjured ribs of the experimental dogs indicated an increase in activation frequency, that is, regional acceleratory phenomenon. The significant effect of PGE1 on enhancing fracture-induced acceleration of haversian remodeling was doubtful, because of the preexistent biologic differences found in the two experimental groups. Nevertheless, the two groups shared a similar pattern of remodeling activity. The posttraumatic mineralization that declined at the sampled sites need further investigation.