Internode disks of tomato (Lycopersicon esculentum cv. Moneymaker) were shaken in glutamine and sucrose solutions. At low external pH (±5.5), the uptake of these substances was accompanied with K(+) efflux, at high pH (±5.5) with K(+) influx. Low concentrations of external K(+) (2 mmol l(-1)) stimulated the uptake of glutamine, which was strongly inhibited by the supply of high K(+) concentrations (20 mmol l(-1)). The effect of K(+) was particularly pronounced at high pH-values. Addition of CCCP in light reduced the uptake of glutamine to the same level as in the dark, and stopped the K(+) fluxes which coincided with the uptake. A model is presented wherein the movements of K(+) across the membrane are related to co-transport, depending on the membrane potential and the Nernst potential of K(+).