In spite of numerous studies, the minimum level of lead exposure at which "sub-clinical" electrophysiologic abnormalities appear is still under discussion. Furthermore, it has not been clarified whether the electrophysiologic changes are directly related to PbB levels or to duration of exposure. This study was conducted on a group of 62 subjects occupationally exposed to lead with average blood lead levels below 50 microgram/100 ml and durations of exposure of less than 10 years. A reduction of motor and sensory nerve conduction velocities and sensory action potential amplitude of the median nerve was found in the subjects exposed to lead, as compared with a control group. Such abnormalities were already present in workers with the lowest blood lead levels, but were more severe in workers whose blood lead levels had exceeded 70 microgram/100 ml, even if this occurred only once in the last two years. The electrophysiologic changes did not correlate with duration of exposure but occurred very soon after initial exposure to lead.