This paper contains a review of the requirements and design of the Environmental Control and Life Support System (ECLSS) for Space Station Freedom; a review of an ECLSS computer program model developed at Boeing for the complete configuration of pressurized elements or volumes; and some significant computed results from this model showing transient performance for subsystems responsible for Temperature and Humidity Control, Atmosphere Control and Supply, Air Revitalization, and Water Recovery and Management.The model referred to is a comprehensive ECLSS Model which has been developed using the G189A Environmental Control System simulation tool.(1)*. The development and potential application of the Model was presented at the 1989 ICES Conference (2). The Model has since been slightly revised and extensively operated, to obtain information regarding nominal and off-nominal ECLSS conditions onboard Space Station Freedom. Detailed results are presented, which include the following: 1) ECLSS conditions under nominal operation; 2)atmospheric effects of varying the crew member locations throughout Freedom; and 3)overall effects upon ECLSS of an Orbiter docking event, accompanied by changeout of the crew members.The computed results presented here are important in assessing the capabilities of the ECLSS equipment in maintaining acceptable levels of temperature, humidity, O2, N2, and CO2 in the occupied volumes during crew changeout events. These events involve up to six extra crewmen from the Orbiter being located temporarily in the Space Station modules in addition to the normal contingent of eight crew members. These additional crew members impose additional demands on the ECLSS equipment, in order to control the levels of the above quantities. Satisfactory control of all off these quantities was achieved with the possible exception of the partial pressure of CO2. The maximum computed ppCO2 was 4.69 mm Hg during the 107 hour long crew transfer period. This level is higher than the 3.00 mm Hg upper limit for normal conditions, but below the 7.6 mm Hg upper limit for degraded conditions. The ability of the water recovery equipment to cope with additional waste water generated and additional hygiene water used by the extra crewmen was also assessed, and was found to be satisfactory.