The probabilistic risk analysis consisted of a comparison between the mathematical frequency distribution models for the residues in the environment and the dose-response curve for the toxicity tests of the most sensitive species (USEPA, 1999). For this comparison, mathematical models were used to explain the relationship between the pesticide concentration and its probability of occurrence in the field. The effect observed for the test organism with the lowest LC50/EC50 value, based on logit distributions, was also considered as represented by the following equation: