Cytogenetic examinations were carried out in 13 cattle farms, two herds of horses, one stag farm and 13 pig farms in areas with different levels of environmental contamination. The frequency of aberrant cells per 100 mitoses was 3.67 +/- 1.89 in pigs (n = 260) and 4.16 +/- 2.4 in herbivores (n = 497). This is a significant difference (p < 0.01). Ten times higher frequencies of chromatid exchanges were found in pigs. The examined herds were classified into three groups by the level of environmental contamination (satisfactory, impaired and severely impaired environment). Significant differences in aberrant cell counts were recorded between the groups of herbivorous animals. Significant differences in pigs were recorded only between herds in satisfactory and severely impaired environments. Significantly higher frequencies of aberrant cells were found in farms of herbivorous animals in the industrial area of Pardubice compared with findings in the South Moravian agricultural area (4.7% and 3.1% respectively). The effect of local contamination sources on farm environment was also investigated. A cattle farm located in the vicinity of a large chemical plant was examined five times at 6-month intervals. An autumn examination revealed significantly higher frequencies of aberrant cells compared with the spring examination.