1. We have studied the characteristics of the abnormal properties of damaged myelinated fibers (conduction velocity > 2.0 m/s) after peripheral nerve injury in a novel in vitro model of the rat sciatic nerve/dorsal root ganglion/dorsal root (L4-5) preparation removed from control naive or sham- operated rats and animals that had received sciatic neurectomy 12-24 days before the in vitro study. A total of 122-245 filaments were recorded in each dorsal root. The proportion of Aα,β and Aδ fibers were not significantly different between control, sham operated, and axotomized nerves. Spontaneous activity was recorded in 3.4% (Aα,β) and 4.6% (Aδ) of fibers in comparison with 0.4% (Aα,β) and 0.3% (Aδ) in naive controls. 2. A sporadic, irregular, low-frequency (<1 Hz) firing was seen in 26% of the fibers with spontaneous activity. Periodical (irregular) bursting pattern was observed in 43% of spontaneously active fibers, whereas a relatively stable, ongoing firing pattern (median frequency: 7.1 Hz) was displayed by 31% of active fibers. 3. Mechanosensitivity of the neuroma/peripheral nerve was displayed in preparations from lesioned [axotomized: 18.2% (Aα,β) and 14.1% (Aδ), sham operated: 2% (Aα,β) and 0% (Aδ)], but not control naive animals. There was no correlation between the presence of spontaneous activity and mechanosensitivity in single fibers. 4. The principal site of spontaneous activity generation was the dorsal root ganglion. Transection of the peripheral nerve (or removal of the neuroma), while recording from dorsal root filaments, produced a cessation of firing in 21% of fibers firing with ongoing discharge. The remaining active fibers continued firing until the DRG was removed. A sustained injury discharge was observed in damaged fibers but not control, undamaged fibers from naive animals after acute peripheral nerve transection. 5. We present an in vitro model for the study of abnormal primary sensory activity in peripheral neuropathy. Although our data are consistent with in vivo electrophysiological findings in published reports, the proportion of damaged afferent fibers displaying spontaneous activity was significantly lower under in vitro conditions. This model may serve as a valuable tool for further physiological and pharmacological studies of peripheral neuropathy.