This chapter examines and compares the acute toxicity in mice and dogs of inhibited methylene chloride, methyl chloroform, and a commercially available paint remover. The introduction of halogens into the molecule of hydrocarbons exerts a pronounced effect not only on the physicochemical properties, but also on the biological behavior of these organic compounds. Halogen substitution usually intensifies the pharmacologic as well as toxicologic properties of a given compound. Both methylene chloride and a paint remover decreased the myocardial contractility and output in the anesthetized open-chest dog preparation. Inhalation of various concentrations of paint remover was associated with various myocardial and hemodynamic changes. The inhalation of paint remover caused an initial increase followed by a decrease in myocardial contractility and cardiac output. The oral route of administration showed no discrimination between the toxicity of methylene chloride and the paint remover.