Foliar absorption of 2,4-D-C14 was rapid in both species. Cotton decarboxylated the side chain of 2,4-D several times faster than sorghum. Both species released C14O2 faster from the carboxyl than from the methylene position. Cotton slowly converted 2,4-D to a chromatographically different material in young leaves and bolls. Sorghum rapidly converted 2,4-D to a product different from 2,4-D and the 2,4-D-complex in cotton. After cotton recovered from 2,4-D-treatment, the regulator could not be detected in subsequent vegetative or reproductive growth.