Effects of diethylstilbestrol (DES) on ion fluxes in oat roots (Avena sativa L.) were investigated by measuring K+ and Cl− absorption and K+ efflux. DES rapidly decreased the absorption of K+ (86Rb) and 36Cl− by excised roots; 10−4 molar DES inhibited Cl− absorption in 1 minute and K+ absorption in 1 to 2 minutes. With a 10-minute incubation period, K+ and Cl− absorption were inhibited 50% by 1.1×10−5 molar and 8.4×10−6 molar DES, respectively. Treatment for 3 minutes with 10−4 molar DES caused irreversible inhibition of K+ absorption. Increasing concentrations of KCl in the absorption media decreased the DES inhibition. Experiments with the DES analogs, DES dipropionate, dienestrol and hexestrol, showed that the steric configuration and the hydroxyl group of the DES molecule are important in determining the inhibitory capacity of the compound.