1. Extracellular single-unit recordings have been made from 295 dorsal horn neurons in the lumbar enlargement of rat spinal cord; 191 neurons in 20 rats with an experimental peripheral neuropathy, and 104 in 10 sham-operated rats. Recordings were made 9-11 days after inducing the neuropathy by tying four loose ligatures around the sciatic nerve in the nerve-injured rats or performing a sham procedure in the sham-operated rats. 2. A survey of the general properties of all neurons encountered was made in the 10 sham-operated rats (104 neurons) and compared with those seen in 17 of the nerve-injured animals (180 neurons). The vast majority (87%; 156/180) of neurons recorded in the nerve-injured animals showed abnormal characteristics; these included responses to very gentle mechanical stimulation of the nerve-injury site and to manipulations that resulted in movement of this site such as extension of the leg and probing of the skin and muscle of the thigh (53%), absence of detectable peripheral receptive fields (RFs; 56%), and very high spontaneous activity (7%). In the sham-operated rats none of the neurons recorded could be activated by gentle mechanical stimulation of the sciatic nerve, and only 6% had no detectable peripheral RF. 3. In the nerve-injured animals, 31% (55/180) of cells had both a peripheral RF, and a response to gentle mechanical stimulation of the nerve-injury site. All cells of this type tested (n = 5) showed very prolonged responses (up to 10 min long) to 15 s pinch stimuli applied to the RF and to 15 s gentle tapping of the injury site. The majority of cells in this group were excited by noxious stimuli (71%; 39/55) and had C-fiber inputs (60%; 33/55). 4. The mean threshold temperatures for evoking responses to heat stimuli in cells tested in nerve-injured rats and in sham-operated animals were not different. However, there was a group of neurons in the nerve-injured rats that had low thresholds, failed to encode stimulus intensity, and did not have a C-fiber input. 5. There were significantly fewer neurons excited by low-intensity stimulation of the skin in the nerve-injured (24%; 43/180) than in the sham-operated rats (71%; 74/104). Measurements of mechanical threshold with von Frey hairs showed that, although the mean threshold did not change, none of the cells tested in the nerve-injured animals had thresholds < 12 mN, whereas the lowest threshold recorded in the sham-operated animals was 0.2 mN.(ABSTRACT TRUNCATED AT 400 WORDS)