Traditionally, the assessment of the effects of radiation on living tissue has been made in terms of absorbed dose. This concept, however, might not be the most appropriate when considering the effects arising from the inhalation of insoluble radioactive particulates in inhomogeneous tissue such as the lung. We have therefore applied microdosimetric methods to this problem and, in particular, investigated in detail how energy depositions are distributed when α particles travel through parenchymal lung tissue. Sections of material derived from rat, beagle and human lung were examined in an image analyzer, and an imaginary plutonium dioxide particulate was placed on the surface of an alveolar sac. The hypothetical α particles emitted from it were followed to the ends of their tracks so that the effects of the material's real structure could be followed in detail. It was found that, taking such detail into account, the α particles traveled much greater distances than might have been thought on the basis of a uniform, structureless lung. It was also found that the specific energy distributions can cover several orders of magnitude and can differ significantly between tissue as a whole, cells and nuclei at low exposures. Attempts are made to correlate these results with recently published data on beagle dogs that had inhaled graded exposure levels of plutonium dioxide aerosols.