The rate of Na(+)/Na(+) exchange as measured with (24)Na(+) in Na(+)-rich cells of Chlorella pyrenoidosa is governed by a single rate constant and saturates with increasing external Na(+) concentration. The K mvalue for this process is 0.8 mM Na(+) and the maximum rate of exchange in illuminated cells is about 5 pmoles cm(-2) sec(-1). These values contrast with a K mof 0.18 mM K(+) and maximum rate of about 17 pmoles K(+)·cm(-2)·sec(-1) for net K(+) influx. Although the Na(+)/Na(+) exchange was only slightly sensitive to light it was inhibited by the uncouplers CCCP and DNP and by the energy transfer inhibitor DCCD. This inhibition of the rate of Na(+)/Na(+) exchange was not accompanied by a loss of internal Na(+). Both the effect of external K(+) on (24)Na(+) influx into Na(+)-rich cells and the inhibition of net K(+) uptake by the presence of external Na(+) indicates that Na(+)/Na(+) and K(+)/Na(+) exchanges share the same carrier and that the external site of this carrier has a three to four times higher affinity for K(+) over Na(+).