The aim of the present study was to examine the effects of nerve growth factor on the healing of unsplinted fractured ribs. After fracture of a rib in male rats, nerve growth factor was delivered by a miniosmotic pump to the fracture site for 7 days at the rate of 1.4 micrograms/day. Callus catecholamine concentrations, bone callus size, histomorphometry, and biomechanical properties of the repairing rib were measured at 7, 21, and 42 days after fracture. After 21 days, concentrations of norepinephrine and epinephrine were significantly increased in the group treated with nerve growth factor compared with those in the control group (211% norepinephrine and 322% epinephrine). Also, the midline longitudinal area of non-osseous (fibrous tissue and cartilage) callus of the fracture was significantly smaller (54%) and had a higher proportion of cartilage in the treated group than in the controls. By 42 days, there was only bony callus between the fracture ends in both the control group and the treated group. The treated group, however, again showed significantly elevated concentrations of norepinephrine and epinephrine (286 and 382%, respectively) and significantly elevated breaking stress (50%) and Young's modulus (51%), together with a reduction in the transverse cross-sectional area of the repair site (57%). The resultant increases in effectiveness and rate of repair of bone with administration of nerve growth factor suggest that it may play an important role in the healing processes of fractured bone.