A considerable amount of integrated Environmental Control and Life Support System (ECLSS) analysis has been performed and documented for the proposed habitable Space Station. Earlier analytic activities have resulted in highly refined models simulating Temperature and Humidity Control (THC) and Atmosphere Revitalization (AR) hardware. As the mechanisms by which these items affect the Space Station environment have become better understood (along with the effects due to operation of various Man Systems utilities), the next stage of the integrated analysis task has been accomplished; i.e., the simulation of the Atmosphere Control and Supply (ACS) subsystem.The focus of the present paper is upon the ACS function in the overall life support system. Modeling of the ACS is unique among the life support disciplines in that it requires accurate representation of all other ECLSS subsystems that interact with the cabin atmosphere (which has now been achieved) in order to be realistic. The G189A Environmental/Thermal Analysis computer program continues to be the preferred analytical tool for this Space Station integrated ECLSS analysis task.As the restructured Space Station design matures, the ECLSS analytic effort is marked by progressive enhancements, both in terms of new modeling features and of an increase in the fidelity of existing features. One modeling enhancement which is covered in detail in this paper is the simulation of equilibrium between pressurized elements which are connected via open hatches. The primary ACS function which is assessed via this enhancement is the makeup of oxygen and nitrogen. The results presented herein clearly demonstrate the integrated effects upon the ACS due to operation of the THC, AR, and Man Systems hardware. The results also delineate the performance of the ACS hardware with respect to specific Space Station program requirements.