Research directed at the development of a Closed Ecological Life Support System (CELSS) has been initiated by NASA. Such systems will provide life support supplies for long duration missions through a combination of biological and physico-chemical processes which will regenerate potable water, a balanced diet, and breathable atmospheres. Research on one aspect of a CELSS, the stability of atmospheric oxygen and carbon dioxide, indicates that autotroph/heterotroph systems closed to the exchange of gas with the external atmosphere show wide fluctuations in atmosphere composition. The causes of this instability are many and their effects must be mitigated if atmospheric stability in a CELSS is to be realized. It is suggested that control strategies be developed based upon a consideration of gas exchange rates and ratios, and preventing the accumulation of important chemical species in system sinks.