Objective: Mutations of an axonal voltage-gated sodium channel (Nav1.7) are linked to either chronic painful diseases or to pain insensitivity. Thus, testing the influence of sodium channel blockade on axonal excitability of different human C-fibers classes is of high interest in the context of their contribution to neuropathic pain. Methods: We recorded C-fibers from the peroneal nerve in healthy human subjects using microneurography. Based on sensory and axonal characteristics, we classified units as: mechano-sensitive afferents (CM); mechano-insensitive afferents (CMi); sympathetic efferents (symp). Lidocaine (Lido; 0.1%, 100ml) was injected intradermally into receptive fields of the C-fibers beneath the electrical stimulation site. Conduction velocity (cv) and activity-dependent cv slowing were assessed before and after injection. Results: Conduction block occured in 87% of fibers and lasted up to 20 minutes. After recovery, cv was reduced in all C-fibers. Activity-induced slowing at low frequencies (1/8-1/4-1/2 Hz) was markedly diminished and partially reversed toward relative speeding only in C-afferents (CM: before Lido: 102.6+0.4%; after: 99.2+0.4%; n= 20; CMi: before Lido: 107.7+0.6%, after: 104.8+0.6%; n= 8; p< 0.01). There was no block preference for CM or CMi fibers (p= 0.9). Total slowing at 2 Hz stimulation significantly decreased (27.1+2.7% vs. 17.6+2.2 %; p<0.01) and 59 % fibers displayed initial relative speeding after Lido in 17 CM units. In contrast, little reduction in 2Hz-evoked slowing and no initial speeding were observed after Lido in 4 CMi fibers. None of the stimulation paradigms induced significant changes in slowing pattern of 9 sympathetic efferents. Conclusion: We bring direct evidence for the key role of sodium channels in the mechanisms of activity-dependent conduction slowing in human C-fibers. Differential lidocaine effects probably reflect differences in sodium channels expression and/or resting membrane potential.