During the 1990s, Japan began experiencing demographic changes that are larger and more rapid than in other OECD countries. These demographic changes will become even more pronounced in future years. We are interested in understanding the role of lower fertility rates and aging for the evolution of Japan's saving rate. We use a computable general equilibrium model to analyze the response of the national saving rate to changes in demographics and total factor productivity. In our model aging accounts for 2 to 3 percentage points of the 9 percent decline in the Japanese national saving rate between 1990 and 2000 and persistently depresses Japan's national saving rate in future years.