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Measurements are reported of 13 C 12 C ratios for air CO 2 at different heights in two Amazonian rain forests. CO 2 emitted from the forest floor is severely depleted in 13 C which produces isotopically light source air throughout the forest. Air δ 13 C values vary very little with height above ground, but are about 5 to 6‰ more negative than the open atmosphere. CO 2 recycling under the canopy depletes all leaf δ 13 C values by a like amount. Additional factors further deplete leaf δ 13 C values by 4 to 5‰ at ground level; this effect decreases with height to zero in the upper canopy, yielding a gradient in δ 13 C values.