A successful CELSS (Controlled Ecological Life Support System) design must accommodate the potential mismatch between the crew's relatively constant CO2 production and the widely varying crop CO2 consumption over the plant growth cycle. Any additional changes in material flows, processor characteristics or other system characteristics may have deleterious effects which propagate throughout the CELSS. Important transient conditions which the system design and planting regimen must allow for are described, including: Crop startup. Crop failures. Changes in number of humans supported. A general-purpose life support system simulator was used to evaluate several CELSS design and operation approaches. The simulator was used to investigate CO2 generation and removal interactions occurring between the CELSS food production subsystem and the rest of the system. These interactions were selected because they are major drivers of the system design and operation.