14CO was supplied to bean leaves in light or darkness at 200–360 ppm in air. In light, CO was converted mainly to sucrose and proteins. The distribution of14C among products suggested that most of the absorbed CO was reduced and incorporated into serine and this was converted to sucrose. Some CO was oxidized to CO2, part of which may have been refixed as such in photosynthesis, but this was not the major pathway of light CO metabolism. In darkness, CO was absorbed nearly as fast as in light but was almost completely converted to CO2and released.Light CO fixation by leaves of a number of species was measured using 1–10 ppm CO in air. Rates varied between 0 and 0.25 μmole/dm2per hour. Rates were roughly proportional to CO concentration, but were unrelated to rates of photosynthesis. CO reduced or abolished CO2fixation in some leaves. CO fixation by vegetation of intermediate CO-fixing capacity is calculated to be in the order of 12–120 kg/km2ground per day, which approaches rates found for soil using much higher CO levels.