This revised Test Guideline 412 (TG 412) has been designed to fully characterize test article toxicity by the inhalation route following repeated exposure for a limited period of time (28 days), and to provide data for quantitative inhalation risk assessments.  It was updated in 2017 to enable the testing and characterisation of effects of nanomaterials tested. Groups of at least 5 male and 5 female rodents are exposed 6 hours per day for 28 days to a) the test chemical at three or more concentration levels, b) filtered air (negative control), and/or c) the vehicle (vehicle control). Animals are generally exposed 5 days per week but exposure for 7 days per week is also allowed. Males and females are always tested, but they may be exposed at different concentration levels if it is known that one sex is more susceptible to a given test article. This guideline allows the study director the flexibility to include satellite (reversibility) groups, bronchoalveolar lavage (BAL), lung burden (LB) for particles, neurologic tests, and additional clinical pathology and histopathological evaluations in order to better characterize the toxicity of a test chemical.