1. To obtain information about structural events that occur in axons, changes in light scattering from squid giant axons were measured during action potentials and voltage‐clamp steps. 2. The scattering changes were measured at several scattering angles. Because the changes in scattering divided by the resting scattering were between 10−6 and 10−5, signal‐averaging techniques were used to increase the signal‐to‐noise ratio. 3. The scattering changes during the action potential were different at different angles. Two types were found, one at 10–30° (forward angles) and the other at 60–120° (right angles). 4. At forward angles, there was a transient scattering decrease during the action potential. The time course of the change was similar to that of the action potential; this change was thought to be potential‐dependent. 5. At right angles, there was a transient scattering increase during the action potential followed later by a second, longer‐lasting increase. Indirect evidence indicated that neither component could be totally potential‐dependent. 6. To further analyse these effects, scattering was measured during voltage‐clamp steps. The changes seen during hyperpolarizing steps were presumed to be potential‐dependent; again two different changes were found, one at forward angles and one at right angles. 7. The potential‐dependent change at right angles occurred with a time course that could be approximated by a single exponential with a time constant τ = 24 μsec. The change at forward angles required two exponentials, τ1 = 23 μsec, τ2 = 900 μsec, to represent its time course. 8. The size of both potential‐dependent changes was proportional to the square of potential. The change at right angles, but not that at forward angles, was increased in size by the addition of butanol or octanol to the bathing solution.
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