The effects of pulsed micro-electrical currents on internal remodeling in the cortex of long tubular bone were evaluated by the following three experiments. 1. Electrodes were inserted in both femora of 14 adult mongrel dogs, 15 mm apart, and pulsed micro-electrical current was applied in the right femoral cortex for 4 weeks, but not in the left femur, which was left as a control. Dogs were divided into 4 groups; in each of these groups current with 1Hz-10 microA, 0.1 Hz-10 microA, 50 Hz-10 microA and 1Hz-20 microA was applied. The effects were evaluated by histometric parameters, i.e. number of resorption cavities (Ar), osteons with osteoid seam (osAf), mineralization rate of osteoid seam (Mo), and perimeter of osteoid seam (Sf). Number of Ar and osAf increased. Bone formation rate (Vf) which is the product of osAf, Mo and Sf increased, especially in the group in which current with 1Hz-10 microA was applied. The main reason for increase of Vf was considered due to that the activation frequency in internal remodeling increased by electrical stimulation. 2. A metal plate was placed on the right humerus, not on the left humerus, both femora of 5 dogs, and electrical current of 1Hz-10 microA was applied in the right femur for either 12 or 16 weeks. Decrease of internal remodeling tended to take place in the mid-portion of the plated area of femur, whereas Vf increased by pulsed micro-electrical currents. Decrease of internal remodeling thus caused by placing a plate and screws increased by pulsed micro-electrical current. 3. Number of osteons in the newly formed bone in the osteotomized gap and in the cortex adjacent to the gap of femora of 7 dogs, which were plated for either 4 or 6 weeks, was measured in longitudinal sections labelled by tetracycline. The number of osteons increased more in the right femur in which current of 1Hz-10 microA was applied than in the left femur. Based on the results above described, it was concluded that bone healing was enhanced by pulsed micro-electrical currents.