Mechanics of breathing as studied in patients with different stages of silicosis in rest and during exercise show the following: dynamic compliance and work against elastic and viscous resistances could not be found to be deincisive indices of exertion dyspnea. Evidence of exertion dyspnea could only be achieved by means of the ration VT/PT (VT = tidal volume, PT = tidal esophageal pressure). Exertion dyspnea was observed at less than 0,081 VT per cm H2O PT In accordance with Campbell and Howell, dyspnea is seen as an inappropriateness between ventilation and pressure needed