Eight Macaca fascicularis monkeys received rapid maxillary expansion with conventional and magnetic appliances. The histomorphometric analysis (SEM) showed increased external root resorption (ERR) in: multirooted teeth, buccal root surfaces, mesiobuccal roots and apical zones. ERR is determined by the impulse, the decrease in the periodontal width and the extent of mineralization of adjacent hard tissues. The microscopic examinations of light, UV and SEM defined two types of cellular cement in ERR areas: a) unfunctional repair cement (slow type), b) functional repair cement (rapid type). The first was characterized by the absence of Sharpey's fibers and by overlapped incremental lines, the second by the formation of new Sharpey's fibers and discriminated incremental lines. These incremental lines indicate five sequential phases with diverse rates of cement apposition, i.e. lag-, incipient-, peak-, steady-, and retreating phase. Therefore, the study recommends the application of a fixed retainer immediately subsequent to rapid maxillary expansion which is followed by an intermittent retention appliance.