Toxicologists and pathologists are often faced with the dilemma of categorizing changes observed in the respiratory tract of laboratory animals as either "adaptive" or "toxic." However, it is often difficult to interpret the nature of a given change as either "adaptive" or "toxic." Certain lesions or changes in the respiratory tract are to be expected from the concentration of materials given or the experimental design of a study. Careful analysis suggests that some of these changes may be more properly described as adaptive rather than toxic within the context of a given study or situation. Tissue changes discussed in this paper include squamous metaplasia of laryngeal epithelium, goblet cell change in respiratory epithelium, macrophage accumulation within alveoli, and bronchiolization of alveolar epithelium. Examples provided show that some of these changes observed in inhalation studies are similar in severity but slightly increased in frequency over sham control animals. The introduction of exogenous material into the respiratory tract of laboratory animals in an experimental setting should be expected to result in certain changes. The challenge scientists must accept is to interpret these changes so that toxic events may be separated from adaptive changes. In order to meet this challenge, studies incorporating several species and novel technologies may have to be utilized.