Conductance changes associated with the response of the squid giant axon have been studied at two temperature ranges (26–27°C.; 9–10°C.) and with modified concentrations of sodium and potassium in the medium. The phase of "initial after-conductance," during which the membrane resistance increases above the resting value, is smaller at the lower temperature. At both temperature ranges it is diminished by doubling K+ in the medium and enhanced by removal of K+. Halving the Na+ of the medium also enhances this phase when K+ is absent, but not otherwise. The time course of the conductance changes alters in form with changes of the external medium. These changes indicate independent changes in the complex of ionic events associated with the response. The experiments therefore confirm the reality of the phase of increased membrane resistance. The magnitude of this change appears to be considerable and requires a transient decrease in the mobility and/or concentration of ions in the membrane. The possible cause of this decrease is discussed.

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