The main goal of this paper is to formulate and analyze an adaptive dynamic mathematical model of the investment behavior of a typical joint-stock firm, which may facilitate the understanding of the influence of investment incentives on the level of its investment activity. The behavior of the model is analyzed by means of Portryagin's maximum principle in four steps, which are successive approximations of the complicated investment behavior of real firms. The analytical solutions of the model are partially interpreted in analyzing influences of the investment incentives, which were introduced into the Czech economy in the framework of the new tax system, on the level of investment activity of joint-stock companies formed in the first wave of voucher privatization