Man's occupancy of space depends not only on his tolerance to hostile conditions existing there, but also, on how much of his natural environment he can control, contain, and take with him. Past experiments have demonstrated that man's ability to function in sealed environments for long periods is directly dependent on, among other factors, the reliability of his life support system in-supplying food, water, a breathable atmosphere, and suitable temperature control. Several possible methods are available for supplying the space traveler with life support. The most obvious, and the only one used to date, is the nonregenerative or socalled canned system. This system requires that all food, oxygen, waste storage facilities, etc. be placed in toto on board the spacecraft prior to launch. The quantity of each support item in the system is determined by the mission profile (duration). The weight of a nonregenerative system increases more or less linearly with each added increment in mission duration. At some mission time, weight and volume considerations will necessitate regeneration (re-use) of some or all of the support system components. If excretory and CO2 wastes are recycled into food, 02 and potable water, initial requirements for weight, volume and power will be substantially increased. However, these requirements will reach a maximum and remain relatively constant, even with further demands in mission duration. The pay-off point between utilization of regenerative and nonregenerative systems has been subject to much speculation. The relationship can be simply illustrated by plotting arbitrary units of weight against mission duration (Figure 1). Values for the systems will intersect at some point in time which has been variously calculated to be between about 30 and 300 days of life support. Estimates vary widely due to misinterpretations, incomplete analyses and well-recognized difficulties encountered in extrapolation from theoretical models and experimental data. Various regenerative systems, chemical and biological, have been suggested and investigated for CO2 removal, 02 generation, and nutritional support in closed ecologies. This paper will be limited to considerations of biological systems and, in particular, to the mass culture of algae and duckweed. Myers and Brown (10) recently reviewed and compiled theoretical considerations and available information on the use of algae for gas exchange and food production. The purpose of this paper, therefore, will be to 1. discuss methods employed in studies on algae and duckweed and 2. illustrate what