p1. The activity of the Nasup+/sup pump in an Nasup+/sup-rich yeast was compared with that in an Nasup+/sup-rich frog sartorius muscle, and found to be very similar to it over the first hour if both were immersed in fluid containing 104mm-Nasup+/sup plus 10mm-Ksup+/sup. 2. The efflux of labelled Nasup+/sup from an Nasup+/sup-rich yeast into an Nasup+/sup-free medium was investigated. In this Nasup+/sup-free medium, Lisup+/sup or choline replaced the Nasup+/sup, and the efflux–content curves obtained with either of these ions were very similar. The curves were sigmoid, reaching or approaching a saturation at the higher internal Nasup+/sup concentrations. 3. The curves obtained with yeast resembled those similarly obtained with frog sartorius muscle by Keynes & Swan (1959), Mullins & Frumento (1963), Harris (1965) and Keynes (1965). The slope of the plot of the logarithm of the Nasup+/sup efflux against the logarithm of the Nasup+/sup concentration in the cells reached its highest value at an internal Nasup+/sup concentration of 15m-equiv./kg. (27m-equiv./l. of cell water). 4. The effect of external Ksup+/sup concentration on the efflux–content relationship was examined. An increased Ksup+/sup concentration was found to increase the Nasup+/sup efflux by raising the saturation value, which is similar to observations made by Harris (1965) with frog muscle. 5. The effect of increasing the external carbon dioxide concentration was investigated. No effect on the slope of the plot of the logarithm of the Nasup+/sup efflux against the logarithm of the Nasup+/sup content was noticed even when the yeast suspension was equilibrated with 100% carbon dioxide. There was, however, a decrease in the amount of Nasup+/sup efflux on equilibrating the solution with carbon dioxide./p
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