Detopped root systems of Ricinus communis plants were used for the study of the effects of temperature and DNP on the uptake of K and Na ions supplied as KNO3 and NaNO3.When K and Na ions were offered together in equivalent concentrations, the steady state uptake rates for K(+) and Na(+) at 23 to 25° gave a K(+)/Na(+) ratio of 3. Increasing the Na(+) concentration relative to K(+) 3-fold did not alter the preferential uptake of K(+). The uptake of K(+) was more sensitive to temperature in the range 10 to 40° and to the application of DNP at 1.5x10(-4) M than was the uptake of Na(+). When NaNO3 was the only salt supplied Na(+) uptake became more sensitive to DNP than when both K(+) and Na(+) nitrates were supplied. Prolonged application of DNP led to net K(+) efflux from the roots, even when no K(+) was being supplied to the roots. Net Na(+) efflux under the influence of DNP occurred only in roots previously grown on Na-containing nutrient medium.The different responses of the K(+) and Na(+) uptake processes to temperature and DNP suggest the operation of different uptake mechanisms for K(+) and Na(+) These results have been considered in relation to the recent concept of dual mechanisms for the absorption of alkali cations by plant tissues.