The membrane resistance of internodal cells of Nitella translucens increased by 50 per cent during the first 5 h after insertion of two microelectrodes into the vacuole even when precautions were taken to eliminate external disturbances. The insertion of a third microelectrode into the cytoplasm did not affect the resistance. In artificial pond water the final value for the plasmalemma resistance was 112 kω cm2 and that for the tonoplast was 6 kω cm2. The increase in the membrane potential after the first hour was less than 10 per cent. A recent suggestion that accurate measurements of the plasmalemma resistance can be made with a microelectrode outside the plasmalemma, but in close contact with it, is criticized. Tests were made of the claim that leakage of current at the point where microelectrodes enter the cytoplasm gives rise to a local increase in current density at the tonoplast and hence leads to an overestimate of the tonoplast resistance. Values for the tonoplast resistance obtained when the cytoplasmic microelectrode was inserted through the plasmalemma were similar to those observed when it was pushed across the cell and inserted through the tonoplast at a point remote from the postulated current leakage. Furthermore, the tonoplast resistance stayed remarkably constant when the plasmalemma resistance varied in a way which would cause different proportions of the applied current to pass through the leak resistance and produce variations in the apparent tonoplast resistance. It is concluded that published values of the tonoplast resistance are not grossly inaccurate.