Inhaled substances can support behavior by acting as reinforcing stimulus events. The deliberate inhalation of volatile materials is attributable to the positively reinforcing properties of these substances and can induce profound toxicity. On the other hand, inhaled substances can also be aversive, e.g., corrosives, certain solvents, and combustion products. Both positive and negative reinforcing properties of inhaled materials can be used to support the behavior of laboratory animals. Several rules of evidence should be met, however, to demonstrate conclusively that an inhalant has such properties. Such properties should be considered in industrial hygiene and environmental quality decisions.