The influence of calcitonin on the healing of a fracture of tibial diaphysis in young rats has been studied with the aid of 85Sr investigations. The animals were subdivided into four groups (with facture + calcitonin, with fracture and no calcitonin, without fracture but with calcitonin treatment and intact controls). During the experimental period up to the day 38, repeated applications of calcitonin and 85Sr along with measurements of different parameters followed. These parameters consisted of: total body weights, 85Sr contents in the fractured and opposite tibiae, total body radioactivity of 85Sr, the weights of dried bones dissected after the day 38, the weights of bone ashes and the specific activity of 85Sr in the ashes. Increased 85Sr-uptake was seen as a sign of increased mineralisation of the callus. Calcitonin suppressed the resorption of bone during the callus formation and enhanced the mineralisation of callus to some extent. The main influence of calcitonin was seen to reduce the whole-body metabolic response to a localized skeletal injury--the fracture. The possible importance of this finding for the clinical practice is discussed briefly.