This method provides information on health hazard likely to arise from short-term exposure to a test chemical by inhalation. It is a principle of the method that only moderately toxic concentrations are used so that ‘evident toxicity’, rather than death/moribundity is used as an endpoint, and concentrations that are expected to be lethal are avoided. Groups of animals of a single sex are exposed for a short period of time to the test chemical in a stepwise procedure using the appropriate fixed concentrations for vapours, dusts/mists (aerosols) or gases.  Further groups of animals may be tested at higher concentrations in the absence of signs of evident toxicity or mortality at lower concentrations. This procedure continues until the concentration causing evident toxicity or no more than one death/ moribund animal is identified, or when no effects are seen at the highest concentration or when deaths/ moribundity occur at the lowest concentration.  A total of five animals of one sex will normally be used for each concentration level investigated. The results of this study include: measurements (weighing at least weekly) and daily detailed observations, as well as gross necropsy. The method provides information on the hazardous properties and allows the substance to be classified for acute toxicity according to the Globally Harmonised System of classification and labelling of chemicals.