Acetylene decreased root and nodule respiration, as measured by CO(2) evolution of nodulated or non-nodulated Glycine max. An inhibition of 25 to 35% in 15 to 30 minutes occurred when 13% C(2)H(2) was introduced in the gas flux which aerated the root nutrient solution. When the light intensity was doubled to 800 microeinsteins per square meter per second, the inhibition increased to 50% and nodule acetylene reduction activity was inhibited 50%.