The uptake, translocation, and metabolism of pendimethalin (PROWL^®) and trifluralin(TREFLAN^®) in cotton, (Gossypium hirsutum) redroot pigweed (Amaranthus retroflexus) and seedling johnsongrass (Sorghum halepense) were determined. The better cotton selectivity of pendimethalin versus trifluralin appears to be due to differences in metabolism and sequestration of pendimethalin in the lysigenous glands. Cotton roots metabolized pendimethalin but did not metabolize trifluralin. Redroot pigweed and johnsongrass were also able to metabolize pendimethalin, but only pigweed metabolized trifluralin. However these two weeds species absorbed 1.3 and 2 times more pendimethalin than trifluralin, respectively. This increased uptake compensated for increased metabolism such that the internal concentration of pendimethalin was much higher the concentration of trifluralin. This higher uptake of pendimethalin occurred in spite of 1.6 to 2 times higher adsorption of pendimethalin than trifluralin to the soil.