The initial rate of uptake of glycine by the tumour cells was measured as a function of the Na(+) and K(+) concentrations in the solution in which the cells were suspended. When [Gly] was 1mm or 12mm, the rate in the absence of Na(+) was independent of [K(+)] and about 3% or 10% respectively of the rate when [Na(+)] was 150m-equiv./l. 2. The Na(+)-dependent glycine entry rate, v, at a given value of [Na(+)] was successively lowered when [K(+)] was increased from 8 to 47 to 96m-equiv./l. A kinetic analysis indicated that K(+) competitively inhibited the action of Na(+). The results were in fair agreement with previous determinations of the kinetic parameters. 3. The presence of 2mm-sodium cyanide and 10mm-2-deoxyglucose lowered the cellular ATP content to less than 3% of the value in the respiring cells. Although v was then about 50% smaller, the relative effects of K(+) and Na(+) on the system were similar to those observed during respiration. 4. A theoretical analysis indicated that the variation of v with [K(+)] is not a reliable guide to the extent to which the K(+) gradient between the cells and their environment may contribute to the net transport of glycine.
