Ultrastructural changes of nerve fibers have been studied in the rabbit abdominal vagal nerves after unilateral vagotomy. Only one side of the abdominal vagal nerve trunk was cut and the opposite side was left untouched. The electron microscopic observations were performed on the ultrathin sections of tissues removed from the two sides of the abdominal vagal nerve trunk and the following results were obtained: 1) Changes in the distal stump of the cut nerve. Axonal swelling was seen from 12 hours to 3 days after the operation. In the swollen axons, accumulations of organella such as mitochondria, myelinfigures and vacuoles were observed. The axons were atrophied but Schwann cells were hypertrophied 4 days after the operation. Seven days after the operation, the axons had disappeared and only the Schwann cells remained. 2) Changes in the proximal stump of the cut nerve. An initial ascending degeneration of the axon was completed 4 days after the operation. The regenerating axons appeared 7 days after the operation. Generally, they were smaller in diameter (0.3-0.4μ) than the normal (0.6-1.3μ). Twenty-eight days after the operation, there were two relatively distinct types of axons; one of them of normal diameter (0.6-1.3μ) and the other had a smaller diameter (0.3-0.5μ). 3) Changes in the intact nerve. a) Two to seven days after the operation, smaller axons, 0.2-0.5μ in diameter and contained microtubules, appeared intermingled with normal axons. In some parts of the unmyelinated fibers, bundles of small axons, each axon being about 0.2μ in diameter and not invaded by Schwann cell processes, were observed. Those bundles of axons were considered to be an early stage of nerve sprouting. b) Seven to twenty-eight days after the operation, in a longitudinal section, some axons appeared as composed of an alternate arrangement of wide and narrow areas. c) Twenty-eight days after the operation, an increase in the number of axons per Schwann cell unit was observed. It was assumed that this increase in axons was the result of nerve sprouting.