The potential genotoxicity of nitrates and nitrites-contaminants of drinking water that have been implicated in carcinogenesis-was investigated in this study. Sister chromatid exchanges and frequency of chromatid/hromosome aberrations were studied in peripheral blood lymphocytes of 70 children who were 12-15 y of age. These children were permanent residents in geographical areas of Greece, where elevated concentrations of nitrates (i.e., 55.70-87.98 mg/l) existed in drinking water. The control group comprised 20 healthy children who resided in areas with very low nitrate concentrations (i.e., 0.7 mg/l). A significant increase in the mean number of chromatid/chromosome breaks was observed in children exposed to nitrate concentrations that exceeded 70.5 mg/l (p < .01), but there was no significant increase in the mean number of sister chromatid exchanges per cell. The results indicate that chronic administration of elevated concentrations of nitrate in drinking water has the capability of inducing cytogenetic effects.