The effect of increased net foliar K+ accumulation on translocation of carbon was studied in sugar beet (Beta vulgaris, L. var. Klein E and US H20) plants. Net accumulation of recently absorbed K+ was studied by observing arrival of 42K+per unit area of leaf. Labeled K+ was added to give an initial concentration at 2 or 10 millimolar K+ in mineral nutrient solution. Because the newly arrived K+constitutes a small part of the total leaf K+ in plants raised in 10 millimolar K+, export of 42K+ by phloem was negligible over the 2- to 3-day period; consequently, accumulation is a measure of arrival in the xylem. In leaves from plants in 2 millimolar K+, export by the phloem was estimated to be of the same order as import by the xylem; K+ per area was observed to remain at a steady-state level. Increasing the supply of K+ to 10 millimolar caused arrival in the xylem to increase 2- to 3-fold; K+ per area increased gradually in the mature leaves. Neither net carbon exchange nor translocation of sugar increased in response to a faster rate of arrival of K+ over a 6- to 8-hour period. In the absence of short-term effects, it is suggested that K+-promoted increase in synthetic metabolism may be the basis of the increased carbon assimilation and translocation in plants supplied with an above-minimal level of K+.