During prolonged periods of quiescence, the membrane potential of cells in the isolated canine coronary sinus, exposed to normal Tyrode solution containing 4 mM-K, declines to about -60 mV. The nature of the resting potential was investigated, in small strips of coronary sinus tissue mounted in a fast-flow system, by recording the membrane potential responses to sudden changes in the extracellular ionic environment. At extracellular K concentrations ([K]o) from 0 to 64 mM the resting potential was little affected by replacing all but 1 mM of external Cl ions with isethionate and methylsulphate ions. At [K]o levels from 4 to 150 mM the resting potential was reasonably well described by the Goldman-Hodgkin-Katz equation on the assumption that the intracellular K concentration ([K]i) was 155 mM and that the ratio of membrane permeability coefficients for Na and K, PNa/PK, was 0.07. In the presence of a high concentration of acetylcholine or carbachol (greater than or equal to 1 microM), the resting potentials at [K]o levels from 1 to 150 mM approximated K equilibrium potentials (EK) calculated on the assumption that [K]i was 155 mM. At [K]o levels less than or equal to 8 mM replacing most of the external Na with sucrose or Tris caused a substantial hyperpolarization, whereas application of 1-2 microM-tetrodotoxin caused only slight hyperpolarization. A transient hyperpolarization, due to enhanced electrogenic Na extrusion, was recorded on switching back to 4 mM-K following brief exposures to K-free solution; no transient hyperpolarization was recorded in the presence of 5 microM-acetylstrophanthidin. The acetylstrophanthidin itself caused a rapid depolarization of several millivolts. Preliminary conductance measurements made with two micro-electrodes in some smaller preparations indicate that the steady-state current-voltage relationship is N-shaped. We conclude that the low membrane potential of quiescent coronary sinus cells reflects not a low [K]i but rather a relatively high ratio PNa/PK, of about 0.07: the Na ions flow into the cells via predominantly TTX-insensitive pathways and are extruded by the electrogenic Na/K exchange pump, which thereby makes a substantial contribution to the resting potential.