Peslin, R., R. Farré, M. Rotger, and D. Navajas.Effect of expiratory flow limitation on respiratory mechanical impedance: a model study. J. Appl. Physiol. 81(6): 2399–2406, 1996.—Large phasic variations of respiratory mechanical impedance (Zrs) have been observed during induced expiratory flow limitation (EFL) (M. Vassiliou, R. Peslin, C. Saunier, and C. Duvivier. Eur. Respir. J. 9: 779–786, 1996). To clarify the meaning of Zrs during EFL, we have measured from 5 to 30 Hz the input impedance (Zin) of mechanical analogues of the respiratory system, including flow-limiting elements (FLE) made of easily collapsible rubber tubing. The pressures upstream (Pus) and downstream (Pds) from the FLE were controlled and systematically varied. Maximal flow (V˙max) increased linearly with Pus, was close to the value predicted from wave-speed theory, and was obtained for Pus-Pds of 4–6 hPa. The real part of Zin started increasing abruptly with flow (V˙) >85%V˙max and either further increased or suddenly decreased in the vicinity of V˙max. The imaginary part of Zin decreased markedly and suddenly above 95%V˙max. Similar variations of Zin during EFL were seen with an analogue that mimicked the changes of airway transmural pressure during breathing. After pressure andV˙ measurements upstream and downstream from the FLE were combined, the latter was analyzed in terms of a serial (Zs) and a shunt (Zp) compartment. Zs was consistent with a large resistance and inertance, and Zp with a mainly elastic element having an elastance close to that of the tube walls. We conclude that Zrs data during EFL mainly reflect the properties of the FLE.