The ICRP Publication 66 Human Respiratory Tract Model (HRTM) has been applied to calculate dose coefficients and bioassay functions using default values of parameters relating to the material and the subjects. The ICRP Task Group on Internal Dosimetry (INDOS) has developed a guidance document on application of the HRTM in situations where using specific information can improve dose assessments. INDOS is now revising the worker exposure documents (ICRP Publications 68 and 78). Application of the HRTM requires a review of the lung-to-blood absorption characteristics of inhaled radionuclides. Where appropriate, compound-specific absorption parameter values will be derived, and other compounds will be assigned to default Types using current information. Although no major changes to the HRTM are envisaged, this revision provides an opportunity for some refining and updating in the light of experience and new information.