UNLABELLED There has been no consensus on a standard calculational approach regarding the concept of administered activity in ventilation studies involving inhaled radioisotope gas or radioaerosols. This is caused in part by a lack of knowledge regarding the actual activity that enters the lung space (as opposed to activity introduced into the delivery system) and to the extended administration times (t(a)) associated with delivery protocols. METHODS This pamphlet reviewed the three primary ventilation procedures, including rebreathing-system protocols, continuous-flow generator output techniques, and radioaerosol delivery systems. RESULTS For each technique, an analytic expression has been derived for a new parameter called lung administered cumulated activity, A(L)(0,t(a)), which is the cumulated activity in the lungs during the administration phase. In addition, another potentially useful parameter has been defined-the mean administered activity for ventilation procedures, which normalizes the administered cumulated activity in the lung over the administration period and may serve to standardize descriptions of protocols between patients and institutions. Examples are provided that illustrate these new concepts for typical ventilation protocol administration parameters. CONCLUSION The models presented can be employed to evaluate lung administered cumulated activity for use in ventilation dose estimate reports as a function of explicit variables (e.g., spirometer volume, generator output rate, wash-in half-time, administration time). In practice, it is recommended that dose estimate reports be based on measurements of cumulated activity in the lung over the administration period and normalized to this administered cumulated activity.