Isobutane is the third hydrocarbon used as a propellant for aerosol products. In the classification proposed in 1974, isobutane, together with dichlorotetrafluoroethane, belonged to the group of low-pressure propellants of intermediate toxicity. Isobutane occurs usually mixed with propane and n-butane in natural gas. Isobutane can be detected with gas chromatographs equipped with a flame ionization detector. The presence of isobutane in the atmosphere is attributed to natural gas leakage, petroleum gas leakage, or diesel exhaust. Inhalation of threshold concentrations of isobutane, methyl chloroform, and trichloroethylene separately revealed that these three compounds share the property of depressing the myocardial contractility. Isobutane exerts the following pharmacologic profile in the open-chest dog preparation: a)decrease in cardiac output, stroke volume, and stroke work; b) decrease in myocardial contractility; and c) decrease in left ventricular and aortic pressures.