The Arctic mutation (APP E693G) is unique, since it is located within the amyloid-beta (Abeta) sequence and leads to Alzheimer's disease (AD). Arctic Abeta peptides more easily form Abeta protofibrils in vitro, but little is known about the pathogenic mechanism of the Arctic mutation in vivo. Here, we analyzed APP transgenic mice with both the Swedish and Arctic mutations (tg-APPArcSwe) and transgenic mice with the Swedish mutation alone (tg-APPSwe). Intense intraneuronal Abeta-immunoreactive staining was present in young tg-APPArcSwe mice, but not in tg-APPSwe mice. Intracellular Abeta aggregates in tg-APPArcSwe were strongly stained by antibodies recognizing the N-terminus of Abeta, while those recognizing the C-terminus of Abeta stained weakly. The Abeta aggregates inside neurons increased with age and predated extracellular Abeta deposition in both tg-APPArcSwe and tg-APPSwe mice. Senile plaque deposition was markedly accelerated in tg-APPArcSwe mice, as compared to tg-APPSwe mice. We conclude that the Arctic mutation causes AD by facilitating amyloidosis through early accumulation of intracellular Abeta aggregates in association with a rapid onset of senile plaque deposition.