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Department of Physiology, University of Wisconsin, 470 North Charter Street, Madison, Wisconsin 53706, U.S.A.

1. In the crayfish, regions of axonal bifurcation where action potential propagation failed during repetitive activity were visualized using modulation-contrast optics. To determine if a reduced safety factor for conduction at the branch sites has a morphological basis, the geometrical ratio (GR) of axon diameters, where GR = (d₁^3/2+d₂^3/2)/d_p^3/2, was calculated; d₁, d₂, and d_p are the diameters of the two daughter branches and the parent axon, respectively.

2. Values of GR at sites of conduction block were usually slightly higher than 1, indicating a small drop in safety factor; however the values were well below the level at which failure might occur due to an impedance mismatch.

3. Calculated values of GR were generally higher in the most peripheral axon regions where propagation was observed to block most readily.

4. Short collaterals containing numerous synaptic vesicles and apparent release sites were observed to have relatively low values of GR (about 0.8); collaterals containing densely populated mitochondria were characterized by relatively high values of GR (about 1.3).

5. It is concluded that the morphological characteristics of the axon in the branching region are conducive to an inherent low safety factor for conduction, but that they cannot play a major role in block development.