1. The membrane potential of the smooth muscle cells of the rabbit main pulmonary artery amounts to -57 mV, the length constant of the tissue is 1.48 mm and the time constant of the membrane 182 msec. On the basis of the electrical properties of its membrane, this smooth muscle tissue is classified as a single-unit type. During outward current pulses, the membrane shows marked rectification and action potentials can never be generated.2. Tetraethylammonium (10 mM) and procaine (5 mM) depolarize the membrane and increase the membrane resistance. By studying the effect of both substances on the (42)K efflux, it could be concluded that they reduce the K-permeability of the membrane. They also suppress the rectification of the membrane and increase the length constant of the membrane. In the presence of TEA and procaine, a graded response of the membrane can be induced by outward current pulses, but overshoot potentials never occur.3. Noradrenaline, in concentrations between 2 x 10(-8) and 10(-7)M, evokes contraction without depolarizing the membrane. When the concentration is increased above 2 x 10(-7)M, noradrenaline depolarizes the membrane and reduces the membrane resistance. A study of the effect of noradrenaline on the K, Cl and Na fluxes has revealed that it increases the permeability of the membrane for these three ions.4. The tissue concentrations of Na and K are 80 and 38 m-mole/kg wet wt., respectively. The amount of Cl in the cellular compartment was measured by an extrapolation procedure and found to be 13 m-mole/kg wet wt. The extracellular space measured with [(14)C]sorbitol is 550 ml./kg wet wt. and the dry wt./wet wt. ratio 19%. The calculated equilibrium potentials for K, Na and Cl (E(K), E(Na) and E(Cl)) are -83, +59 and -26 mV, respectively. In efflux experiments under steady-state conditions, the following rate constants have been calculated: 0.092 min(-1) for Na, 0.029 min(-1) for Cl and 0.0054 min(-1) for K. The calculated value for the ratio P(Na)/P(K) was 0.22 and for P(Cl)/P(K) 0.63.5. K-free solution and 2 x 10(-6)M ouabain depolarize the cells by about 8 mV. After exposure of the cells to K-free solution, they hyperpolarize on readmission of K, suggesting that part of the membrane potential could be due to electrogenic transport of ions.6. A decrease of external Ca depolarizes the cells and increases the membrane resistance. Na-deficiency hyperpolarizes these smooth muscle cells but this procedure does not prevent the depolarization induced by Ca deficiency.