During a passive exhalation performed between (FRC + VT) and FRC, peak expiratory flow (VmaxE) varies as a function of the respiratory system expiratory resistance, RT, with the following relationship: VmaxE = VT/(RT . CT), where Cr is the total respiratory system compliance (static). This expression assumes: (1) a laminar flow regimen during expiration, and (2) a constant CT value over the range of VT. Both these conditions are usually attained during controlled ventilation in the apneic patient. VmaxE can be monitored continuously with relative ease, and may represent a simple and useful index of the mechanical properties of the respiratory system in the anesthetized patient.