All employees of a chemical plant division producing chlorfenvinphos were studied, i.e. 35 males aged 25-57 years (mean 42.1); their employment period ranged from 1-15 years (mean 9.0). Chronic bronchitis was diagnosed in 13 workers (37.1%). Mean air chlorfenvinphos concentrations in the work environment estimated with gas-liquid chromatography were from 0.0008-0.0018 mg/m³ (maximum allowable concentration according to Polish standards is 0. 01 mg/m³). The activity of erythrocyte acetylcholinesterase was similar to that observed in people who were not exposed to chemicals, however, a slightly lowered activity of plasma cholinesterase in the studied population was evidently the result of mild liver impairment. Spirometric investigations performed in the studied workers revealed slight alterations manifested by increased intrathoracic gas volume (ITGV) (the value of the index was 138.6% of the mean value, 24 workers with an abnormally high index), as well as by decreased specific airway conductance (sGaw); its mean value in the studied group was 58.5% of the mean standard (11 people showed an abnormal index). Substantial functional changes were found in the respiratory muscles. Maximal inspiratory pressures (MIP = 97. 2 +/- 28.3 cm H₂O) as well as maximal expiratory pressures (MEP = 113.9 +/- 44.2 cm H₂O) in the studied group were significantly lower (p<0.01) as compared to those observed in the control group (MIP = 120.7 +/- 31.7; MEP = 154.4 +/- 40.2 cm H₂O) of 22 males having similar cigarette smoking habit, without occupational exposure to chemicals. It was also found that the people who had worked for more than 10 years under conditions of exposure to chlorfenvinphos showed significantly lower (p<0.05) values of maximal inspiratory pressure (87.2 +/- 28.06 cm H₂O, n = 17) compared to the workers whose period of employment was shorter than 10 years (106.6 +/- 26.8 cm H₂O, n = 18). The two groups were comparable with regard to age and smoking habits. The values of maximal expiratory pressures were similar in both groups. No essential disturbances in neuro-muscular transmission were observed; only in 3 workers (8.5%) the electrostimulating myasthenic test showed some disturbances in neuro-muscular transmission. It seems that respiratory muscles impairment in humans exposed to chlorfenvinphos results from changes in the metabolism and structure of muscles, and partly from lung hyperinflation.