AbstractWe present a modified TREE-SPH code to model galaxies in three dimensions. The model includes a multi-phase description of the interstellar medium which combines two numerical techniques. A diffuse warm/hot gas phase is modelled by SPH, whereas a cloudy medium is represented by a sticky particle scheme. Interaction processes (such as star formation and feedback), cooling, and mixing by condensation and evaporation, are taken into account. Here we apply our model to the evolution of a Milky Way type galaxy. After an initial stage, a quasi-equilibrium state is reached. It is characterised by a star formation rate of ∼1 M⊙ yr–1. Condensation and evaporation rates are in balance at 0.1–1 M⊙ yr–1.