Biological systems flown in space, as either experiments or part of the spacecraft habitat Environmental Control/Life Support System, will require controlled environments to optimize results. Plant growth flight experiments have, up to this time, used or planned on using unconditioned crew compartment atmosphere or bottled gases. Use of unconditioned crew compartment atmosphere limits the choice and flexibility of experiments. Bottled gases are heavy and take up valuable space. Life Systems, Inc. sponsored by the National Aeronautics and Space Administration's Kennedy Space Center, Bionetics Corporation, and internal funds has developed a small (115 cm3), lightweight (270 g) unit which controls the two primary atmosphere parameters for plant growth experiments-pCO2 and trace contaminant levels. The unit is capable of providing variable pCO2 outlet levels as specified by the experimenter. Capacity of the unit, depends on specific experiment design parameters e.g., flow rate, inlet and outlet pCO2. The currently designed unit has an operational life ranging from 10 days to greater than 60 days. The unit was successfully flown on board the Shuttle Discovery STS-29 mission as an integral part of a plant growth experiment.