2,4-Dinitrophenol (DNP) induced a rapid, pH-dependent, H(+) influx into slices of maize (Zea mays L.) scutellum. H(+) influx was very slow at pH 5, but it increased rapidly as the pH was lowered over the range 5 to 3.5. H(+) influx was accompanied by a nearly equal K(+) efflux. DNP rapidly accumulated in the slices, reaching a peak after 10-20 min of incubation; and then there was an efflux of DNP which continued for at least 2 h. Nearly constant rates of H(+) influx were obtained over extended periods during which internal and external DNP concentrations were changing. Bathing solution pH had a much greater effect on H(+) influx than it did on the peak tissue DNP level or on DNP efflux. DNP efflux also occurred when H(+) influx was inhibited by CaCl2 or by a pH of 5. It is inferred from these data that DNP efflux indicates a lowering of cytoplasmic pH caused by H(+) influxes not only from the bathing solution but also from the vacuole. It is concluded that H(+) influx is induced, but not carried, by DNP. The H(+) carrier, it is suggested, is a H(+) pump (perhaps an ATPase) which is reversed by DNP.