Disclosed are a method for operating a piezoelectric drive system (10) used for adjusting movable parts (12, 14), especially in a motor vehicle (13), as well as a piezoelectric drive unit (10) comprising at least one piezo motor (12) that is fitted with at least one piezo actuator (18). At least one frictional element (30) of the piezo motor (12) makes it possible to generate a relative movement in relation to a frictional surface (14) located across from the frictional element (30). The at least one piezo actuator (18) is triggered with an excitation signal (93) by means of an electronic unit (42). A response signal (130) of the drive system (10) is detected using the at least one piezo actuator (18), a change in the load of the drive system (10) being recognized as a result of a change in the response signal (130).