With a modified, less time-consuming, Karnovsky-Roots method, this study evaluated the reduction and recovery of rat sciatic nerve cholinesterase (ChE) in degeneration and regeneration. Characteristics of motor fascicles included not only partial staining of acetylcholinesterase (AchE) in the axon, but also a small amount of more intense staining of butyrylcholinesterase (BuchE) in the unmyelinated fibers, scattered between the myelinated fibers in a spot or fleck form. Characteristics of sensory fascicles included more intense staining of BuchE in the unmyelinated fibers, (appearing as irregular balls or pieces), and a scattering between unstained myelinated fibers. Mixed fascicles had characteristics of both motor and sensory fascicles. Unmyelinated fibers exhibited BuchE activity, but myelinated fibers exhibited AchE activity. Preparation time for a specimen to react to ChE-positive staining was about 1 to 2 hr; the method is very suitable for diagnosis during surgery. After nerve transection, AchE activity at the distal nerve end began to reduce gradually; no AchE could be tested after two weeks. But BuchE activity in unmyelinated fibers could be found until 30 days after transection. After epineurial suturing of the peripheral nerve transection, new AchE activity could be found at the anastomosis site until about two weeks and at 1 cm distal to the anastomosis site until about three weeks. It became more intense with the passage of time and, at about six weeks, regenerated AchE-positive myelinated fibers could be seen at the distal end of the nerve.