Abstract Excretion data for any xenobiotic can be a very informative portion of a balance study if one is adequately prepared to interpret it. Things which should be considered are the route of administration, the molecular weight of the parent compound and possible metabolites, the polarity of the parent compound, the possibility of nonpolar metabolites, and the position of the radiolabel if one is present. Given these factors for a xenobiotic, the investigator should be able to determine the efficiency of intestinal absorption, the importance and degree of metabolism by intestinal microbes, and the effect of route of administration on the disposition of the compound by studying the excretion data. In addition, the routes and rates of excretion should be indicative of the types and rates of metabolism to which the parent compound is subject. In the present review of the excretion of insecticides the author has endeavored to provide examples of how various investigators have determined these parameters for different types of insecticides and an effort has been made to point out the various types of pitfalls to which this type of research is subject. It is not unlikely that with further refinement of the techniques, and with the application of adequate kinetic parameters that the excretion of insecticides and other xenobiotics over a short period of time could be used to predict their long-term disposition. Furthermore, if accurate sampling methods could be devised, it might also be possible to use one or more of the minor routes of xenobiotic excretion as a nonintrusive assay of the total body burden of lipophilic xenobiotics. In summary, it is anticipated that complete balance studies for the disposition of insecticides and other lipophilic xenobiotics will be considered an increasingly important part of the information deemed necessary for the formulation of regulations regarding their use. Quantitative data on the excretion of these compounds will be an integral part of these balance studies. If the excretion studies are carefully planned, the data obtained should prove very useful to the interpretation of the in vivo metabolism and disposition of the compounds in question. It is hoped that this present article will be an aid in the planning of these studies.