Sural nerve conduction velocity (NCV) and relative refractory period (RRP) using paired stimuli was performed on 34 healthy smokers and on 34 healthy non-smokers. No smoking probands showed a NCV of 52.2 +/- 4.1 m/sec, the smoking probands 50.7 +/- 4.1 m/sec. The RRP of the first group was 2.6 +/- 1.3%. On contrast to these results the RRP of smoking probands was 5.8 +/- 1.5%. The differences of the RRP between smokers and non-smokers are significant (p less than 0.001 Wilcoxon-test). In order to investigate the persistence of this functional impairment of the sural nerve, 19 healthy probands, who had stopped smoking (R = 0.25-13 years), were examined also. The NCV of this group was 52.2 +/- 4.5 m/sec and the RRP 3.9 +/- 1.9%. The RRP of ex-smokers is different from smoking and non smoking probands (Wilcoxon-test, p less than 0.05). The results of our investigation clearly demonstrate, that smoking induces a slight dysfunction of the sural nerve, which is not completely reversible. This may suggest, that smoking induces for example subclinical changes of the myelin sheath. In general, healthy probands are not asked for their smoking habits. It may be concluded that smoking represents an important and problematical factor concerning the age-dependency of peripheral nerve function.