The effects of newly synthetized derivatives of furylethylene: i) 1-(5-nitro-2-furyl)-2-phenylsulfonyl-2-furylcarbonyl ethylene (FE1), ii) 1-(5-phenylsulfonyl-2-furyl)-2-phenylsulfonyl-2-furylcarb onyl ethylene (FE2), iii) 1-(5-phenylsulfonyl-2-furyl)-2-phenylsulfonyl-2-tienocarb onyl ethylene (FE3), on the reaction kinetics of the dog kidney (Na/K)-ATPase were tested. Besides the conjugated triene moiety of the furylethylene skeleton, the groups responsible for the reaction with nucleophilic groups, the formyl group that connects the second furyl ring in FE1 and FE2 and the formyl group that connects the thienyl ring to the furylethylene moiety in FE3. Among the furylethylenes tested, only FE1 was found to react effectively with beta-mercaptoethanol (beta ME) and glycine (GLY) as model substances containing nucleophilic groups, and also exhibit an inhibitory interaction with the (Na/K)-ATPase. A suppression of the reactivity of the formyl group due to the replacement of the furyl ring with the more aromatic thienyl ring in FE3 did not induce any significant change in the reactivity of the compound with the model substances or with (Na/K)-ATPase. On the other hand, replacement of the NO2 group on the furylethylene moiety (in FE1) by the less electron-attracting phenylsulfonyl group (in FE2 and FE3) yielded a considerable suppression of the inhibitory effect on (Na/K)-ATPase. Moreover, in comparison to FE1, FE2 and FE3 were found to react less potently with the model nucleophilic substances. The results indicated that the conjugated triene moiety on the furylethylene part of the molecule of FE1 may be made responsible for the inhibitory interaction with the nucleophilic aminoacid residue on the (Na/K)-ATPase molecule. FE1 interfered competitively with the (Na/K)-ATPase activation by increasing amounts of potassium. This was manifested by a significant increase in the apparent K0.5App value and a decrease in the apparent cooperativity constant, nApp, for potassium ions, but had no influence on the apparent VmaxApp value for potassium. With respect to the activation of the enzyme with sodium ions and ATP, only FE1 decreased the VmaxApp values while having no considerable influence on the other kinetic variables. It was concluded that FE1 inhibits the (Na/K)-ATPase by selective interaction with some essential nucleophilic (probably SH and/or NH2) aminoacid residues located in, or closed to the potassium binding site of the enzyme molecule.