The localization of 99mTc(Sn)-MDP in bone tissue was compared with 99mTc-MDP by means of microautoradiography of undecalcified bone sections. Sections of good histological quality were obtained by a rapid embedding method in methylmethacrylate. No differences were found in the localization of these radiopharmaceuticals in fetal rat calvariae after incubation in vitro or in rat femora after administration in vivo. In the incubation experiment, hydrolyzed 99mTc was formed. The uptake was high in areas of new bone formation. No uptake was seen in cells or in resorbing areas. In compact bone 99mTc(Sn)-MDP was predominantly taken up in the vicinity of blood vessels.