QUARTERLY JOURNAL OF ECONOMICS rigidity in one part of the system should entail lack of flexibility in A remarkable characteristic of the Harrod-Domar model is that it consistently studies long-run problems with the usual short-run tools. One usually thinks of the long run as the domain of the neo- classical analysis, the land of the margin. Instead Harrod and Domar alk of the long run in terms of the multiplier, the accelerator, "the capital coefficient. The bulk of this paper is devoted to a model of ong-run growth which accepts all the Harrod-Domar assumptions except that of fixed proportions. Instead I suppose that the single composite commodity is produced by labor and capital under the standard neoclassical conditions. The adaptation of the system to an exogenously given rate of increase of the labor force is worked out in ome detail to see if the Harrod instability appears. The price-wage interest reactions play an important role in this neoclassical adjust ment process, so they are analyzed too. Then some of the other rigid assumptions are relaxed slightly to see what qualitative changes result: neutral technological change is allowed, and an interest-elastic savings schedule. Finally the consequences of certain more"Keynes- ian''relations and rigidities are briefly considered IL. A MODEL OF LONG-RUN GROWTH There is only one commodity, output as a whole, whose rate of production is designated Y((). Thus we can speak unambiguously of the community's real income. Part of each instant,'s output consumed and the rest is saved and invested. The fraction of output saved is a constant s, so that the rate of saving is sr(. The com munity's stock of capital K(t)takes the form of an accumulation of is then just th increase of this capital stock dK/dt or K, so we have the basic identity at every instant of time Output is produced with the help of two factors of production capital and labor, whose rate of input is L(t). Technological possi- bilities are represented by a production function Y= F(K, L) Output is to be understood as net output after making good the depre- ciation of capital. About production all we will say at the moment is