ABSTRACTIn this paper we analyze the working of a capitalist system with sophisticated financial institutions, where the Modigliani‐Miller theorem does not apply, so that the financial part of the economy affects the working of the real side. This model, in which the real and the financial sectors are connected throught a simple portfolio approach, and where the ratio between cash flow and debt commitment affects the pace of investment, explains the emergence of instability and, more generally, the cyclical behavior of the economy as an internally generated phenomenon of a capitalist system. As far as the dynamic behavior of the model is concerned, we can envisage the following process: when investment activity is low, the system evolves toward a stationary state; as soon as the accumulation rate starts growing, the model shows the existence of limited cycles and, when the elasticity of investment to internal finance increases further, a chaotic dynamic appears. Finally, we expect a situation of financial crisis to emerge when the elasticity of investment to internal financing constraint goes beyond a certain level, i.e. when the realized flow of profits is not sufficiently high to validate the debt commitments. The working of the model is referred to an economics with no technical change, while its dynamic properties will be investigated by the use of numerical simulatio