In 1978--1980, measurements of dissolved carbon monoxide in seawater were carried out during three ship expeditions in the northern and southern Atlantic Ocean, covering an area between 53 ¿N and 35 ¿S. Measurements in surface water were performed by a continuously working equilibration technique that allowed for the determination of small and rapid variations of the dissolved CO. Carbon monoxide concentrations in the water body increased toward the sea surface, there reaching saturation factors of up to 50 with respect to atmospheric CO. The daily average CO saturation increased with average chlorophyll a concentration and light intensity and decreased with wind speed. The dissolved CO showed strong diurnal variations, with maximum values in the afternoon. Diurnal variations were observed within the entire euphotic zone, indicating the CO is both produced and consumed in the seawater. Carbon monoxide production in seawater was mainly due to photooxidation of dissovlved organic matter. Photobiological and micorbial CO production processes were of minor importance. Carbon monoxide destruction was due to a light-independent microbial metabolism. Based on average CO saturation factors of 30¿20 in the surface water of the ocean, the total source strength for atmospheric CO is calculated to 10--180 Tg yr-1.