Compared to corn (Zea mays L.) (resistant), oats (Avena sativa L.) (susceptible), and giant foxtail (Setaria faberii Herrm.) (susceptible), fall panicum (Panicum dichotomiflorum Michx.) and large crabgrass (Digitaria sanguinalis (L.) Scop.) metabolized 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) at an intermediate rate. The order of tolerance of these five species (corn > fall panicum and large crabgrass > giant foxtail > oats) is identical to the order of their ability to metabolize atrazine. In 6 hr, corn, fall panicum, large crabgrass, giant foxtail, and oats metabolized 96, 44, 50, 17, and 2%, respectively, of the 14C-atrazine absorbed from a 10 ppm solution and translocated to the foliage, leaving concentrations of 2.2, 34.8, 30.1, 59.8, and 66.3 mμ moles, respectively, of atrazine per g of fresh weight of shoots. Hydroxyatrazine [2-hydroxy-4-(ethylamino)-6-(isopropylamino)-s-triazine] was found in the shoots of corn and giant foxtail. Corn shoots also contained a more hydrophilic metabolite, presumably a peptide conjugate. Hydrophilic metabolites found in the shoots of giant foxtail, fall panicum, and large crabgrass were chromatographically identical to the hydrophilic metabolite found in corn.