Ethacrynic acid inhibits anion movements in ox red blood cells. The I50 for chloride is 7 X 10(-6) M. The inhibitory effect is instantaneous and completely reversed by washing the cells with a Ringer solution, suggesting that reaction with a membrane SH group is not involved in this process. Direct proof that ethacrynic acid does not act by its reactivity with thiol groups is given by experiments with dihydroethacrynic acid, a derivative that lacks the ability to combine with SH groups: the characteristics of inhibition are strictly identical (instantaneous and reversible; I50 equals 9 X 10(-6) M). All the phenoxyacetic derivatives tested were also more or less inhibitory. The relative activity of all the derivatives was highly correlated with their liposolubility, indicating that hydrophobic interaction is important in determining drug effect and influence of steric factors is minimal. The data suggest that inhibition essentially results from a hydrophobic interaction between ethacrynic acid and apolar regions of the membrane protein allowing chloride transport.