Potassium (K), an element required slightly lower than nitrogen (N), is one of the essential major nutrients for trees in forests. K deficiency inhibits tree growth and limits forest productivities. Ectomycorrhizal fungi are one of important microorganisms in forests. Many forest trees have evolved mutualistic symbioses with the fungi that contribute to their nutrition and growth. In the fungus-tree associations, the fungi obtain carbohydrates from host trees and, in turn, provide the plants with mineral nutrients such as phosphorus (P), calcium (Ca), magnesium (Mg) and K. In our present experiment, ectomycorrhizal fungi isolated from pine and eucalyptus forests were grown in liquid culture media with various K supplies (26, 130, and 650 mg/L K₂SO₄) and with Ca²⁺ signal and anion channel inhibitors such as Trifluoperazine, TFP; Verapamil, VP; Ruthenium red, RR and Niflumic acid, NIF. The efflux of acetate and protons by the fungal isolates in the response to the K supplies and inhibitors were studied in vitro in order to understand the mechanisms of ectomycorrhizal fungi to improve the nutrition of host plants. The results indicated that the fungal species varied greatly in both growth and nutrient absorption (N, P and K). The hyphae biomass at harvest, for example, showed at least twice difference, ranging from 52.91 mg/flask to 121.72 mg/flask. The reasons for the variations in growth and nutrition could be explained by the biological adaption to soil nutrient environments where they lived for a long time through evolution and selection. Oxalate, acetate, malate, citrate, succinate and large amount of protons were detected in the culture solutions. Taking into the account of dissolution and decomposition of minerals and rocks caused by protons and organic acids, ectomycorrhizal fungi could thus make K in mineral structures available for the host plants. In addition, all study fungi released acetate into liquid culture media and the low K supply stimulated significantly the acetate efflux. The efflux rate correlated negatively with K concentration in culture solutions (r=-0.734, n=60) and fungal K absorption (r=-0.617, n=60). In forest soils, therefore, the influence of K on acetate efflux by the external hyphae of ectomycorrhizas could be physiologically and ecologically important for the improvement of tree K nutrition at low K supply but prevention against K leaching in fertile soils. The inhibitors of Ca signals and anion channels decreased the acetate efflux by ectomycorrhizal fungal isolates in culture solutions with low K supply. It seems reasonable to suggest both Ca signals and anion channels involved the process of the fungal acetate exudation in this case. The primary signal of K deficiencies could act as the intracellular second messenger to react on calmodulin (CaM), by which changed the Ca²⁺ distribution inside and outside of the hyphae cells, and then stimulated the cascade reactions responsible for acetate efflux, including gene expression, acetate synthesis and anion channel activation. Concerning the difference between the ectomycorrhizas in the field and the fungal isolates in vitro, further investigation is necessary to carry out on the mechanisms of the detail processes with ectomycorrhizal trees.