AERIN is a FORTRAN IV program written to simplify the application of equations for computing the organ and tissue burdens and doses resulting from an acute inhalation exposure to a radioactive aerosol. The models which form the basis for the calculations are (1) the deposition and clearance models of the ICRP Task Group on Lung Dynamics, (2) the gastro-intestinal (GI) tract model of EVE, and (3) a single exponential model for the “critical” organs and tissues. The fundamental data for the respiratory and GI tract models are written into the program. The user can select suitable dosimetric parameters for the respiratory and GI tract tissues, choose the appropriate metabolic and dosimetric quantities to be used in calculations for the “critical” organs and tissues, and specify the number of computations to be made arid the time interval between them. The output consists of a complete listing of the assumed parameters as well as the organ and tissue burdens and doses as functions of time post exposure.