While in the field environment, the test item is connected to the vehicle at several interface points. The combined structure is then subjected to a variety of field loads that produce motions at the test item interface and external (non-interface) points. A commonly employed laboratory simulation procedure consists in attaching the test item to a single vibration exciter through a test fixture. Choice of appropriate test fixtures and definition of suitable laboratory test item inputs is of major importance in obtaining realistic simulation of a field environment. MIL — STD810D recommends that: (i) The test item be attached to the exciter through a rigid test fixture; (ii) When available, Field data should be used to define the test item inputs in the laboratory environment. In this case, enveloping techniques are frequently used with the field data in order to define the input that will be applied in the laboratory. The objectives of this paper is to show the consequences of using these current laboratory simulation procedures on the test item dynamic response. The test item, the vehicle, and the test fixture are modeled by a multi degree of freedom (MDOF) lumped systems. Random excitation and response signals are used in the simulation process. The test item laboratory inputs are defined in terms of an acceleration auto spectral density (ASD) that is obtained by enveloping field data as recommended in MIL — STD810D. The test item accelerations obtained in the laboratory when the test item is attached to either a rigid or flexible test fixture are compared with the actual motions that occur when the test item is attached to the vehicle in the field. It is shown that both the rigid and the flexible test fixtures seriously affect the resulting test item accelerations and that the current enveloping techniques expose the test item to excessively high dynamic strain levels.