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1. The effects of phospholipases on resting potential, action potential and membrane ionic conductances of squid giant axons have been studied by means of internal perfusion and voltage clamp techniques. The sample of phospholipases used exhibited both phospholipase A and B activities.

2. When applied externally, phospholipases had no effect on the resting and action potentials at a concentration of 100 µg/ml.

3. When applied internally, the enzymes effectively suppressed the amplitude and maximum rate of rise of the action potential at a concentration of 100 µg/ml. The resting potential started decreasing appreciably only after the action potential had been suppressed to a considerable extent.

4. Under voltage clamp conditions, the peak transient current was suppressed following internal perfusion of phospholipases, 150 µg/ml. The steady-state current began to decrease, and the leakage current to increase only after the transient current had been suppressed to a considerable extent.

5. The time course of sodium inactivation was not significantly slowed by internal perfusion of the enzymes.

6. The curve relating the peak transient conductance to the membrane potential was shifted in the direction of depolarization after internal perfusion of phospholipases. No appreciable shift of the steady-state conductance curve was observed.

7. These experimental results are discussed in connexion with the possible role of membrane phospholipids in the conductance changes associated with excitation.