Cementum surface alterations induced by 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) in the maxillary first molars in rats were studied by means of scanning electron microscopy. Single or triple injections of HEBP inhibit the formation of acellular extrinsic fiber cementum and delay the formation of cellular mixed-fiber cementum. The results indicate the importance of acellular extrinsic fiber cementum as a protection barrier against root resorption and the different mechanisms underlying the formation of the two cementum varieties.