The work aimed at defining the pattern and frequency of rhythm disturbances in basic brain bioelectric activity in EEG recordings, routinely regarded as normal, in workers exposed to chronic carbon disulfide (CS2) poisoning, without detected symptoms of organic lesions in the nervous system. The study covered 188 viscose spinners occupationally exposed to changing concentrations of CS2 vapours whose mean annual results of measurements ranged between 10 and 35 mg/m3, and the mean individual exposure fell within the range of 9.9-41.6 mg/m3 (mean--25.8 mg/m3). The frequency of alpha rhythm was analysed in EEG recordings with exclusive or nearly exclusive alpha rhythm in two groups studied. The presence or absence of subjective symptoms was taken as a criterion for the selection of subjects. Group I was composed of 116 subjects with subjective symptoms. Group II consisted of 72 workers with neither subjective nor objective symptoms. The age in group I ranged from 26 to 65 years (mean, 48.7 years), and the duration of employed from 5 to 39 years (mean, 23.3 years). The complaints reported were of neurasthenic and polyneuropathic type. The age in group II ranged from 28 to 63 years (mean, 49.4 years), the duration of occupational exposure accounted for 8-43 years (mean, 25.6 years). The control group included 194 persons with EEG routinely regarded as normal, matched by gender, age and work shift distribution. The subjects in Group I reported most frequently the following complaints: headache (72.4%), vertigo (48.3%), increased emotional irritability (42.2%), dysmnesia (36.2%), sleep disorders (27.6%), concentration difficulties (25.9%), limb pains (36.2%) and paraesthesia (27.6%). In the groups of subjects chronically exposed to CS2, both with subjective neurological symptoms, and without subjective or objective neurological symptoms, EEG recordings, routinely regarded as normal, did not reveal constant and symmetrical frequency of alpha rhythm waves. EEC recordings in the CS2 exposed workers with subjective neurological symptoms revealed almost constant frequency (12.9%) or slightly fluctuating frequency (41.4%) in 54.3% of subjects. In 45.7% of subjects considerable fluctuation of alpha rhythm frequency at the range of 3-4 Hz was found. Frequency disturbances applied to both sides, however, they were more pronounced in the leftside leads. Statistical analysis showed, neither in Group I nor in Group II, significant relationship between the pattern of alpha rhythm frequency fluctuations, the duration of CS2 exposure and the cumulative exposure index. Substantial and asymmetric fluctuations of alpha rhythm frequency found in the exposed group of subjects with subjective symptoms were considered as subclinical disturbances in the function of the central nervous system.