Uptake and movement of paraquat (1,1'dimethyl-4,4'bipyridylium dichloride) into the transpiration stream of 25-year-old slash pine (Pinus elliottii Engelm.) trees was followed using methyl-14C-paraquat (10Ci/tree) applied in 8 and 3 percent solutions with commercial paraquat. Initial uptake from saturated cellulose pads into xylem was rapid, whereas upward axial movement in outer rings of xylem was 100-fold less than the established velocity of water transport in pines. Velocity of paraquat-11C movement was approximately 30 cm/day vertically and 0.5 cm/day radially along negative pressure gradients in xylem. Recovery of 14C-paraquat by sulphuric acid extraction of xylem samples above the site of application, followed by thin layer chromatography of partially purified extracts, indicated that much radioactivity was still in the form of methyl-11C-paraquat. The physiological significance of paraquat's strong affinity for cellulose, resulting in its slow movement both vertically and horizontally, is briefly discussed as it relates to increased oleoresin synthesis and lightwood formation. Forest Sci. 21:359-364.