In the chemodynamical models of galaxies the energy input from massive stars into the ambient medium results in a self-regulation of the star formation rate (SFR). A thorough analytical and numerical study of this model shows that there is always a strong and negative feed-back, and the SFR becomes almost independent of the assumed stellar birth function (SBF). The time-scale to reach this equilibrium is much shorter than the gas consumption time-scale, hence the models evolve along this solution for most of the time. This mechanism provides a physical explanation for a quadratic dependence of the SFR on gas density. For more details cf. Köppen et al. (1995), A&A 296, 99 and in preparation.