Potentiation and additive effects of pesticides are considered major problems in the determination of safe daily intakes of these chemicals. Examples of potentiation between two organophosphates, such as ethyl-p-nitrophenyl benzenethionophosphonate (EPN) and malathion, delnav and malathion, and guthion and dipterex are considered. Although this type of potentiation could give rise to acute accidents, the most important point in evaluation centres on the possible effects of small amounts of potentiating compounds in the daily diet. Likewise, certain drugs of the phenothiazine type potentiate the toxic effects of organophosphates. A review of the data for both types of potentiation indicates that the consumption of raw agricultural products containing pesticides at tolerance levels constitutes a minimal hazard. A two-year rat-feeding investigation using a combination of eight flavouring agents and six pesticides, at use levels and higher, did not cause a significant alteration of toxicity in comparison with the toxicity of the individual substances.