The K+-stimulated phosphatase activity of microsomes from rat kidney was not inhibited by L-phenylalanine, but the HCO3-stimulated phosphatase activity was markedly inhibited by L-phenylalanine. Valinomycin enhanced the HCO3-stimulated phosphatase activity, but did not enhance the K+-stimulated phosphatase activity. Ouabain did not inhibit the HCO3-stimulated phosphatase activity, but inhibited the K+-stimulated phosphatase activity. The renal K+-stimulated phosphatase activity was suppressed to 40% of the control values by adrenalectomy, but the renal HCO3-stimulated phosphatase activity was little suppressed by adrenalectomy. The renal K+-stimulated phosphatase activity in intact and adrenalectomized rats was found to be significantly elevated, in a manner similar to the elevation of the renal (Na+ + K+)-ATPase activity by aldosterone treatment (P less than 0.02).