Spontaneous activity, and mechanical and thermal sensitivity were investigated in regenerating afferent nerve fibers within 4-21 days post sural nerve lesion (crush or transection and resuturing) in anaesthetized rats. About 33-40% of the myelinated (A) and 22-27% of the unmyelinated (C) fibers excited by electrical nerve stimulation exhibited at least one of these ectopic discharge properties. In total 177 A- and 169 C-fibers with ectopic activity were analysed. Most A-fibers (161/177) were mechanosensitive. Spontaneous activity (median 1 imp/s) was present in 23/177 and thermosensitivity in 14/177 A-fibers (13 of them being activated by heat stimuli). Almost all A-fibers (159/177) exhibited only one type of ectopic discharge property. Most C-fibers (94/169) were thermosensitive responding either to cold (n = 45) or to heat stimuli (n = 33) or to both (n = 16). Eighty-four of 169 C-fibers were spontaneously active (median 0.3 imp/s) and 75/169 C-fibers were mechanosensitive. Both the proportion and the discharge rate of spontaneously active C-fibers were significantly higher after crush than after section and resuturing of the nerve. About 60% of the C-fibers (101/169) had only one ectopic discharge property and 40% two or three. In conclusion, regenerating cutaneous afferent A- and C-fibers may develop mechano- and/or thermosensitivity as well as spontaneous activity. We suggest that spontaneous and evoked ectopic activity in regenerating cutaneous afferents are a function of the intrinsic functional properties of these neurons and of the interaction between the regenerating nerve fibers and non-neural cells during Wallerian degeneration in the nerve distal to the nerve lesion.