Electron probe microanalysis for K and Cl and enzymic determination of malate were performed on epidermal strips of Vicia faba L. which had been incubated with 0.1 equivalent of K(+) per liter in the absence or presence of Cl(-). In the absence of Cl(-), iminodiacetate, a presumed impermeant zwitterion, served as anion. With no Cl(-) in the medium, 91% of the K(+) imported into the guard cells during stomatal opening was neutralized by malate production; import of Cl(-) (presumably from the rest of the epidermal tissue) contributed 6%. In the presence of Cl(-), 50% of the necessary negative charges were provided by malate synthesis, 45% by Cl(-) import. Stomatal opening was not obviously affected by the chloride concentration in the incubation medium, but malate production declined roughly linearly with the logarithm of [Cl(-)] between 10(-5) and 10(-1) equivalent per liter.