CIFI (Circulation dans un Immeuble des Fumdes de l'incendie) is a computer program based on a multi-room zone model. It has been developed at CSTB for the purpose of predicting air and smoke movement in a multi-storey building. In the previous versions of CIFI, the mass flows circulating from room to room through vertical openings were calculated from the differences in vertical pressure fields across these openings, but no estimate of entrainments was made. We introduced recently in CIFI a simple set of criteria in order to decide on the possibility of distributing each flow through a vertical opening between the two gaseous layers located downstream. This results in plumes and entrained flows into these plumes, which were approximately estimated. A description of our approach in given and applied to the interpretation of experiments at NBS.