Membrane potential of cells in the isolated rabbit coronary sinus was measured by conventional glass microelectrode and investigated the effect of [K⁺]₀ variation in control, 20 mM and Ach-containing Tyrode solution. The results obtained were as follows: 1) The resting membrane potential exposed to normal Tyrode solution containing 3 mM K⁺ was about -60 ~ -65mV. At extracellular K⁺ concentrations from 1 to 30 mM the resting Potential was reasonably well described by Goldman -Hodgkin -Katz equation on the assumption that [K⁺]₁ was 150 mM and that the ratio of membrane permeability coefficient for Na⁺ and K⁺, P_Na/P_K (α) was 0.07. 2) In 20 mM Na-Tyrode solution (replacing by equimolar Tris) the resting membrane potential was hyperpolarized by 15 to 20 mV and showed slightly deviated to depolarized direction compared to the predicted value by Goldman-Hodgkin -Katz equation. 3) In the presence of 10^-6M Ach, the resting potentials at [K⁺]₀ levels from 1 to 30 mM were well fitted with the predicted value on the assumption that P_Na/P_K was 0.0144. It could be concluded that the low resting membrane potential of coronary sinus cells reflects a relatively high ratio P_Na/P_K of about 0.07.