Rubidium uptake in potassium-starved cells followed biphasic kinetics in the micromolar and millimolar range and was independent of the temperature. In contrast, Rb+ uptake in normal-K+ cells followed a monophasic kinetics in the millimolar range and increased at temperatures higher than 30 degreesC. Differences in the K-m, values and in the Arrhenius plots of Rb+ uptake suggest different uptake systems in K+-starved and in normal-K+ cells. In addition, the substantial inhibition of Rb+ uptake caused by carbonyl cyanide-nz-chlorophenyl hydrazone indicates that these systems are strongly dependent on membrane voltage. Lithium (sodium) tolerance, influx, and efflux were separately studied. F. oxysporum was shown to be very tolerant to sodium, while lithium caused a specific toxic effect. Li+ uptake in K+-starved cells exhibits a monophasic kinetics with low affinity. Li+ afflux was not affected by external pH or addition of potassium to the medium, suggesting that a Na+/cation antiporter is not involved in this process.