Ethylene is a potent plant hormone. Actively growing crop plants produce ethylene throughout development and this accumulates in the air of closed plant growth chambers. Ethylene levels in the field rarely rise above 1 ppb (0.001 ppm), but levels in closed plant growth chambers at NASA-KSC routinely increase to 0.05 to 0.1 ppm. Reduced seed set and even complete sterility have been associated with elevated ethylene levels, but no controlled studies have been done to clearly implicate ethylene in the poor seed set. We examined ethylene levels from 1 to 20 ppm on Super-Dwarf wheat in replicate growth chambers and compared the effects to control chambers without ethylene. Plants grew well at 20 ppm ethylene but plant height decreased as ethylene increased. There was no effect on the rate of development as indicated by the time of heading. Seed set and yield were excellent in the control chambers, but plants in all the ethylene chambers were completely sterile. In a second study, effects of 0.25, 0.50, 0.75, and 1.0 ppm ethylene on USU-Apogee wheat were compared to a control chamber. A single treatment receiving 1.0 ppm ethylene prior to anthesis and no ethylene following boot stage was also tested. As in the previous study, there was a linear decrease in plant height with increasing ethylene. Longitudinal leaf rolling (into cylinders) was visually apparent at all ethylene levels as compared to flat leaves on control plants. Leaf rolling is a sensitive indicator of ethylene pollution in closed chambers. Seed set was reduced at all ethylene levels except the pre-anthesis treatment. These studies are an essential prerequisite to the design and sizing of ethylene scrubbing equipment for bioregenerative life support.