1. Potassium and sodium concentrations within the Pacinian corpuscle fluid have been investigated in adult cats. The sodium and potassium contents of samples were estimated by means of integrative ultramicroflame photometry. 2. The analytical results showed the values of the potassium concentration in the Pacinian corpuscle fluid and in the blood plasma to be 6.19 +/- 0.72 and 2.78 +/- 0.38 mequiv/1, respectively. 3. The increasing excitability of the Pacinian corpuscle in potassium-rich solution was shown by electrophysiologic methods. In potassium-free solution two phases of changing receptor excitability were observed. 4. Removal of sodium from external solution reduced the receptor potential to 10% of its original value. 5. Some mechanisms of the increase in excitability of primary tissue mechanoreceptors and secondary mechanoreceptors of the sense organs are discussed.