Abstract Metabolic events following the addition of the phytotoxin syringomycin were characterized in cell suspensions of the yeast Rhodotorula pilimanae and maize callus cells. The toxin induced K + efflux, a cessation in the acidification of the suspension medium, and a change in oxygen consumption. In yeast cells, an increased electric field over the plasma membrane was found as measured by enhanced tetraphenylphosphonium (TPP +) uptake. At high concentrations of the toxin, the electric field increase was transient, the medium acidification was reversed, and the respiration rate was lowered. Restriction of the oxygen supply of the cells allowed the separation of two syringomycin-induced TPP + uptake phases. The first component, observed with a minimal supply of oxygen, was accompanied by a net influx of protons and K + efflux. The second component was observed with subsequent aeration of the cell suspension and coincided with the reestablishment of medium acidification. We postulate that a transport protein (probably a H +/K + antiporter) in the plasma membrane is activated allowing an electrogenic K + efflux down its concentration gradient and a simultaneous H + influx. In addition, an energy dependent proton flux from the cytoplasm to another cell compartment is postulated to account for the second TPP + uptake phase.