Toxicological implications due to the use of herbicide-contaminated drinking water, as well as other organic chemicals, are related to their nature and levels. These implications can be defined for each substance on the basis of an adequate evaluation of epidemiological information and experimental data on animals. In this paper, World Health Organization's procedures for establishing guidelines for 11 herbicides widely used in Italy are described. Furthermore, data and information about the use of these herbicides and their levels in Italian drinking-water supplies are also reported and discussed. Finally, factors and conditions responsible for the groundwater contamination by some herbicides in determined areas are presented and discussed.