In Alzheimer's disease, amyloid-␤ peptide aggregates in the extracellular space to form senile plaques. The process of plaque deposition and growth has been modeled on the basis of in vitro experiments in ways that lead to divergent predictions: either a diffusion-limited growth model in which plaques grow by first-order kinetics, or a dynamic model of continual deposition and asymmetrical clearance in which plaques reach a stable size and stop growing but evolve morphologically over time. The models have not been tested in vivo because plaques are too small (by several orders of magnitude) for conventional imaging modalities. We now report in vivo multiphoton laser scanning imaging of thioflavine S-stained senile plaques in the Tg2576 transgenic mouse model of Alzheimer's disease to test these biophysical models and show that there is no detectable change in plaque size over extended periods of time. Qualitatively, geometric features remain unchanged over time in the vast majority of the 349 plaques imaged and re-imaged. Intervals as long as 5 months were obtained. Nonetheless, rare examples of growth or shrinkage of individual plaques do occur, and new plaques appear between imaging sessions. These results indicate that thioflavine S-positive plaques appear and then are stable, supporting a dynamic feedback model of plaque growth. Key words: amyloid; transgenic; Alzheimer; two-photon; in vivo imaging; senile plaque; microgliaAggregates of the amyloid-␤ peptide (A␤) form senile plaques, one of the classic neuropathological lesions of Alzheimer's disease. In vitro observations show that A␤ is extremely insoluble and precipitates to form aggregates (Hilbich et al., 1991), and that exogenous A␤ decorates existing plaques and enlarges them according to first-order kinetics (Jarrett and Lansbury, 1993;Esler et al., 1996). These observations led to a model in which a nidus is formed and plaques then grow in a time-and concentrationdependent manner (Jarrett and Lansbury, 1993;Esler et al., 1996).Paradoxically, however, the size distribution of senile plaques appears to remain constant rather than increasing with increasing duration of illness (Hyman et al., 1993). Together with observations on the fine structure of plaques, these data led to a statistical physics-based dynamic feedback model in which a feedback process was postulated to limit plaque growth, leading to stable plaque size and, in principle, the shrinkage of existing plaques (Hyman et al., 1995;Cruz et al., 1997; Urbanc et al., 1999a,b). Distinguishing between these possibilities is important for understanding the life history and pathophysiology of amyloid deposition in Alzheimer's disease, but it has not been possible because direct measurements of plaques in vivo have not been achieved.Tg2576 transgenic mice overexpress a mutant form of the human amyloid precursor protein and develop senile plaques in an age-related fashion, with amyloid-␤ deposits occurring first at ϳ8 -10 months of age (Hsiao et al., 1996;Irizarry et al., 1997).The number of plaques...