A histomorphometric study was carried out on bone samples in the region of healing defects in the tibias of beagles of various ages. A 5 mm diameter drill hole defect was made in the mid-shaft of the tibia. Eleven of the beagles received either vehicle (n = 6) or prostaglandin E2 orally (n = 5) for the 30-day period from surgery to time of necropsy. Ten dogs received local injections of vehicle (n = 4) or prostaglandin E1 directly into the defects for the first 10 days after surgery. Double labels were given with each of two fluochrome markers, calcein prior to surgical treatment, and oxytetracycline hydrochloride prior to 30-day sacrifice. The regional remodeling changes were evaluated in 40-50 micron thick cross-sections taken 2 cm proximal to the defect and matching samples from the contralateral side. In the controls, the changes were variable and reflected primarily increased formation on the surfaces of the cortex. Cortical endosteal bone formation, as indicated by oxytetracycline labeling, was increased in both control groups and there was an increase in labeling in the periosteal and haversian envelopes as well, in the local injection control group. In the dogs given prostaglandin E2 orally, there was increased periosteal bone formation in addition to increased cortical-endosteal formation. When healing sides were compared to controls, prostaglandin E2-treated dogs also had increased osteoid formation in all three envelopes and increased resorption surface in the cortical endosteal envelope, indicating accelerated remodeling.(ABSTRACT TRUNCATED AT 250 WORDS)