Foliar penetration of 14 C-difenzoquat (1,2-dimethyl-3,5-diphenyl-1-H-pyrazolium), in the presence of a surfactant, into both wild oat (Avena fatua L.) and barley (Hordeum vulgare L. ‘Conquest’), was rapid and continued at a nearly constant rate for 72 hr, the maximum time period studied; 93 and 84% of the applied dose was absorbed by the leaves of wild oat and barley, respectively. Added surfactants at 0.01% to 0.3% and high relative. humidity (RH) greatly enhanced the penetration of the herbicide into wild oat. An increase in temperature from 10 to 30 C resulted in a large increase in penetration. Commercial formulations of bromoxynil (3,5-dibromo-4-hydroxy-benzonitrile) + MCPA {[(4-chloro-O-tolyl)oxy] acetic acid} (1:1, w/w), 2,4-D[(2,4-dichlorophenoxy)acetic acid] ester and barban (4-chloro-2-butynyl m-chlorocarbanilate) markedly increased the penetration of 14C-difenzoquat into wild oat. Amine formulation of 2,4-D, on the other hand, had no effect on penetration. Foliarly-applied 14 C-difenzoquat rapidly translocated mainly in the acropetal direction. Most of the absorbed 14C, however, was accumulated in the treated area and a few centimeters above the treated area. There was some basipetal movement, and 24 hr after treatment 14C was detectable in the roots. Root uptake of 14C-difenzoquat was very rapid, but upward movement of 14C was slight, and even 24 hr after treatment most of it was retained by the roots. Chromatographic analysis of plant extracts up to 15 days after treatment with 14C-difenzoquat revealed no evidence of difenzoquat metabolism by wild oat, barley, and wheat (Triticum vulgare L. ‘Thatcher’). The selective action of difenzoquat in wild oat and barley cannot be accounted for by differences in its foliar penetration, translocation, or metabolism in these species.