Objective: To analyze quantitatively the circumaxillary suture opening after alternate rapid maxillary expansions and constrictions (Alt-RAMEC). Materials and Methods: Twelve inbred cats were randomly grouped into two equal groups for 1 week of rapid maxillary expansion (RME) (1 mm/day) or 5 weeks of Alt-RAMEC (1 mm/day). At the end of the experiment, the craniofacial skeleton of each cat was harvested. Each circumaxillary suture was then probed at three sites with a 0.5-mm pointed periodontal probe. A smooth probing without penetration was an ineffective suture opening (Ͻ0.5 mm), while a probing with penetration was an effective suture opening (Ͼ0.5 mm). For each suture, the quantity of suture opening (%) was the effective suture opening/(effective ϩ ineffective suture opening). The intergroup differences were analyzed by chi-square test (P Ͻ .05). Results: Five weeks of Alt-RAMEC opened the circumaxillary sutures significantly more than 1 week of RME. This affected the circumaxillary sutures running coronally and articulating directly to the maxilla (56.9% vs 36.1%, P Ͻ .001), the sutures running sagittally, but articulating indirectly to the maxilla (94.4% vs 64.8%, P Ͻ .001), and the sutures running coronally, but articulating indirectly to the maxilla (58.3% vs 33.3%, P Ͻ .01). The sutures running sagittally were opened significantly more (94.4%-100.0%) than those running coronally (56.9%-58.3%), no matter if they articulated directly or indirectly with the maxilla. Conclusions: Alt-RAMEC opens both the sagittally and coronally running circumaxillary sutures quantitatively more than conventional RME. However, more than 5 weeks of Alt-RAMEC would be needed to increase the opening of the coronally running circumaxillary sutures. (Angle Orthod. 2009:79; )