A method has been developed for studies of the influence of plants on atmospheric carbon monoxide. The method has been used for extensive laboratory experiments with two plants, Vicia faba and Platanus acerifolia. The experiments show a light dependent CO-production by plants corresponding to an average production rate per cm2 of leaf area of 3¿10-13 g/sec for a radiation intensity of 5¿104 erg/cm2 sec. Assuming this production rate to be representative of global conditions the world wide annual CO-production by plants is estimated to be 0.5¿1014 g.