In a previous study we found an increased rate of chromosomal aberrations in substation workers. To follow up this finding we in this study present data from 38 employees of electric power companies; 19 of the subjects worked with the repair and maintenance of circuit breakers and disconnectors in 400 kV-substations. The other 19 served as controls and were only exposed to normal environmental electromagnetic fields. Coded blood samples were sent to a laboratory for determination of the rate of chromosomal aberrations (CA), sister chromatid exchanges (SCE), and cells with micronuclei (MN). Compared to the control group the exposed men displayed a statistically significant increase in CA and cells with MN. No increase was found in the frequency of SCE. Since "in vitro" studies of lymphocytes exposed to transient electric currents (spark discharges) produced similar results the increase in chromosomal damage in substation workers may be associated with exposure to transient electric currents during work.