The tails of Sprague-Dawley rats of approximately 50 g body mass were either left straight or bent to form a loop containing three or five vertebrae (CV). Subsequent treatment was as follows: (a) in situ: segments were removed at 0, 8, 16, 24 h, 3, 5 and 7 days and examined histologically: (b) transplants: skinned segments were transplanted autologously and examined at the same time intervals; (c) normal saline: skinned segments were placed in a 0.9% solution maintained at (i) 4 degrees C and (ii) 37 degrees C and examined 8, 16 and 24 h later. The results show that on bending a bone in situ the remodelling which occurs reflects the displacement of the soft tissues, and more specifically the periosteum, towards or away from the bone surface. Functionally the developing periosteum consists of three zones and when stressed its fibroelastic component moves away from or towards the osteogenic layer either applying tension to it stimulating bone formation, or pressure eventually inducing bone resorption. These changes are mediated via the mid-zone. The effects on the fibroelastic component alone are best seen in transplants or in normal saline at 37 degrees C where the osteogenic layer dies. The periosteum in growing bones is ideally structured to respond rapidly and sensitively to altered strain by initiating adaptive surface remodelling of bone.