Pesticides are registered in Canada under the authority of the Pest Control Products Act, administered by the Department of Agriculture. Pre‐market registration requirements include submission of extensive toxicology data in order to allow evaluation of potential human hazards which may be associated with exposure to pesticides. Drinking water guidelines in Canada, although under provincial jurisdiction, are generally recommended by the federal government with collaboration of provincial authorities. In 1987, 42 pesticides were selected for development of guidelines for drinking water. When complete, the 42 pesticides will represent approximately 80% of all agricultural pesticides utilized in Canada, based on active ingredient weight sold. As do many other regulatory agencies, the Department of National Health and Welfare uses two different approaches to derive health‐based guidelines or maximum acceptable concentrations of pesticides in drinking water. The threshold approach assumes that there is a dose below which no adverse effect will occur. This threshold level is closely related to the experimentally determined no observed adverse effect level and, when divided by an appropriate uncertainty or safety factor, forms the basis for the establishment of the maximum acceptable concentration. The second approach is the non‐threshold or risk assessment approach and this assumes that there is some probability of harm at any level of exposure. This latter approach is generally utilized for chemicals believed to be capable of causing cancer and has not been used for any of the existing 29 pesticide drinking water guidelines developed in Canada.