1. The effects of low temperature on conduction in single myelinated and non-myelinated axons of the feline saphenous nerve were examined and compared. Nerves were cooled by a conventional thermode, but thermal gradients were minimized by an insulating layer of agar-saline gel over the nerve and the face of the thermode.2. The mean blocking temperature of thirty-one non-myelinated axons, 2.7 degrees C, was significantly lower than that of 111 myelinated axons, 7.2 degrees C. No evidence for a differential block of myelinated axons according to their normal conduction velocity could be demonstrated.3. Reductions in the proportional conduction velocities of both myelinated and non-myelinated axons were nearly identical between 17 and 37 degrees C. However, below 17 degrees C the rate at which the proportional conduction velocity of the non-myelinated axons fell during cooling was significantly less than for the myelinated axons and was sufficient to account for their lower blocking temperatures. As a result, critical minimum conduction velocities were reached at higher temperatures in myelinated axons than in non-myelinated axons.4. The conduction velocity of successive impulses in a train slowed progressively to a constant value which depended on the frequency of stimulation. Consequently, the early impulses were separated by intervals that exceeded those between stimuli and were not affected by temperatures that blocked later impulses. The pattern of block was consistent with an increasing refractoriness of the axons as the temperature fell.5. The maximal frequency of discharge that myelinated axons could carry at temperatures between normal and 12 degrees C was related directly to fibre size. Non-myelinated axons could conduct low frequency trains of impulses at temperatures that blocked such activity in myelinated axons. In all axons, high frequency trains of impulses could be completely blocked at temperatures which permitted lower frequency trains to pass uninterrupted.6. Hysteresis in the blocking temperatures of axons was related to the hysteresis in their conduction velocities.