1. Eighty-eight intrafascicular neural recordings were obtained in 10 normal subjects, 5 patients with axonal degeneration and 11 patients with demyelinating neuropathy. 2. Stimulus levels required for perception and fibre activation were higher in neuropathic subjects. Fibres transmitting touch perception had significantly lower conduction velocities in both patient groups, but were very much lower in the group with demyelinating neuropahty than the group with axonal degeneration. Maximum electrical stimulation evoked dispersed fibre responses in the axonal degeneration group and more dispersed, slowly conducting fibre potentials in the demyelinating group. In patients with hypertrophic Charcot-Marie-Tooth disorder, usually only a small group of slowly conducting low amplitude potentials was recorded. 3. Delivery of a train of supramaximal stimuli caused prolongation of latency and dispersion of fibre potentials in all microneurographic recordings. The changes were significantly greater in the axonal neuropathy group than in normals, and recovery was slower. The demyelinating neuropathies showed significantly greater changes than both the normal and the axonal neuropathy groups, and post-tetanic conduction slowing became even more marked after limb temperature was raised. 4. Surface SAP recordings showed normal refractory period in chronic axonal neuropathy but significant latency prolongation occurred in demyelinating neuropathy. 5. It is concluded that both receptor and nerve fibre abnormalities contribute to sensory dysfunction in degenerative and demyelinating neuropathies.