Long-term electrophysiological cross-talk between nerve fibers has been demonstrated in rat sciatic nerve following induction of an amputation neuroma. Experiments wee designed to establish an anatomical basis for this phenomenon. The sciatic nerve was transected and the epineurium oversewn with 10-0 nylon in 16 adult male Sprague-Dawley rats. The resulting neuromas were prepared for ultrastructural analysis 7, 14, 30, and 60 days later. An analysis of the unmyelinated nerve fibers showed normal configuration of the fibers, with normal organelles, separated by Schwann cell processes in the neuroma. However, degenerating unmyelinated nerve fibers and nerve fibers with masses of neurofilaments were often observed. In approximately 10% of the total population observed, there were two or more unmyelinated nerve fibers in a single Schwann process fascicle. Some of the multiple unmyelinated nerve fiber fascicles had nerve fibers that were in membranous apposition. This configuration of unmyelinated axons could form the anatomical basis for long-term physiological cross-talk between axons in a neuroma, and could be of consequence in pain production.