The O2 dependence of net H(+) efflux of maize coleoptiles has been investigated. Below 100 μM O2, H(+) efflux in young (1 cm long) coleoptiles is markedly decreased while old (7 cm long) coleoptiles show a decline only at 10 μM O2. Old coleoptiles show the same decrease in net H(+) efflux as young ones if treated with fusicoccin. The ratio of alteration of CO2 production to the change in net proton efflux is about 1:1 at 40-80 μM O2 but not at 10 μM O2. An influx can be observed at 10 μM O2 in young as well as in old coleoptiles if the H(+) concentration is held at values below pH 6.5. Lower O2 concentrations lead to an increase of net H(+) efflux, which might be caused by leaching of organic acids resulting from anaerobic processes, but CO2 production is not significantly changed at these values. It is proposed that more than one system is responsible for proton translocation across the plasmalemma. One of the systems has a high sensitivity to reduced O2 concentration which is within the same range as the high Km of the alternative path.