Soybean (Glycine max L. Merr.) seedlings were inoculated with Glomus mosseae (GM) and Bradyrhizobium japonicum (BJ) together or separately to study the effect of interactions on net H(+) effluxes of nodules or extraradical hyphae by in vivo vibrating electrode techniques. GM promoted three-fold the H(+) effluxes of nodules on mycorrhizal lateral roots and BJ increased eight-fold the net H(+) effluxes of hyphae developing in the vicinity of nodules on lateral roots. Increments in plant P content were positively and linearly correlated with the net H(+) efflux of nodules and hyphae. It is concluded that increased H(+) effluxes of nodules resulted from enhanced nitrogenase activities induced by the presence of the AM fungus in lateral roots. The results point to additive effects of interactions between mycorrhizal fungi and rhizobia in increasing the extent of acidification of the "nodulesphere" and the hyposphere.