The membrane potential, electrical impedance and 86Rb+ efflux rate constants were measured in the rat lens perifused at 35°C. The membrane conductance was obtained from the difference between the magnitude of the impedance at low (〈 0·1 Hz) and high (〉 100 Hz) frequencies. Values of the rubidium permeability coefficients (PRb) were obtained from the rate constant and potential data. The values for the membrane potential and conductance in control solution (5 mM potassium) were ‐69·6 mV and 5·5 x 10−4 S respectively, while the computed permeability was 2·9 x 10−8 m.s−1. On perifusing with 35 mM potassium, the membrane depolarized by 25 mV and the conductance and rubidium permeability increased considerably. These increases could be blocked by quinine (0·3 mM), tetraethylammonium (30 mM) and 4‐aminopyridine (10 mM). The latter agent was more effective at alkaline pH (8·3). It is suggested that there are voltage‐gated potassium channels that are inhibited by these three agents. After the initial depolarization in high potassium, there was little further change in membrane potential with any of the inhibitors. All three agents, however, produced a marked depolarization when applied in control solution. This was accompanied by a decrease in conductance and rubidium permeability, suggesting that, in the rat lens, some voltage‐gated potassium channels are activated at the resting potential.