The cellular effects of fluoride (as F – from NaF) on Na + and K + transport across the rat erythrocyte membrane were investigated using 22 Na as a tracer and nonradioactive Rb + as a K + congener. Exposure of rat erythrocytes to 20 mM NaF for 30–120 min considerably diminished (by 68%) Rb + influx into the cells, comparable to the inhibitory action of 1 mM ouabain. F induced a modest accumulation of cellular Na + and a loss of cellular K + significantly greater than the effects produced by ouabain. Removal of Ca 2+ from the standard medium resulted in a significant decrease of F-induced Na + accumulation and 22 Na uptake and almost completely suppressed F-induced loss of K +. Treatment with 1 mM amiloride and 0.2 mM bumetanide inhibited the F-induced Na + influx by 70% and 20%, respectively, suggesting an involvement of amiloride-sensitive pathways (Na + /H + exchanger or Na + channels) and Na-K-2Cl – cotransport. Addition of 50 µM Al 3+ (activator of G- proteins) into incubation medium did not affect the F-induced alterations of both Na + influx and intracellular Na + and K + content. Treatment of the cells with 1 mM quinine or 1 mM Co 2+ (blockers of Ca 2+ transport), 1 mM amiloride, or 5 µM KN-93 (inhibitor of Ca 2+ /calmodulin-dependent protein kinases) produced substantial inhibition of F- induced Na + accumulation by 40–48% and K + loss by 16–30%. These findings suggest that the observed changes in activity of a number of enzymes and intracellular Ca + concentration may mediate many of the early events involved in the response of rat erythrocytes to inorganic F.