This experiment was aimed at illuminating the relationship between electromagnetic and mechanical stimuli of bone formation when present simultaneously. Movable and stationary intramedullary wire implants were studied in rabbits treated with a pulsing electromagnetic field (PEMF) 4 h/day for 3 weeks, and were compared with identical control animals without PEMF. Trabecular bone formed routinely at spontaneously movable implants, but not at stationary ones. On average, PEMF-treated movable implants in the femur induced 44% more bone than untreated movable implants. Also, in the PEMF-treated femora, a 22% enlargement of the area of the medullary canal was observed compared with no-field controls. In the tibia, these effects were weak or nonexistent. The PEMF used did not induce bone at stationary implants, suggesting that under these conditions it is not a primary trigger in vivo.