1. The differential sensitivity of saphenous nerve fibres in the cat to block by procaine HCl was re-examined by recording identifiable unit action potentials from small nerve filaments.2. Small myelinated axons were blocked more quickly than large myelinated axons, but this differential effect could not be accounted for by differences in anaesthetic concentration requirements.3. The onset of block in non-myelinated axons was slower than or equal to that of small myelinated axons depending on anaesthetic concentration.4. Absolute differential block of non-myelinated and small myelinated axons was obtained by limiting the length of axons exposed to procaine to 2 mm.5. Differential rates of blocking among myelinated axons appear to depend on differences in the length of axons that must be exposed to blocking concentrations of procaine and to arise from the irregular distribution of such concentrations within an exposed nerve.