ABSTRACT The problem of determining the relative importance of the G.I. tract and the respiratory system as portals of entry of aerosols into the systemic circulation following inhalation exposure was studied by obtaining data on the kinetics of uptake and retention of various inhaled and ingested fission product aerosols. Exposure of mice toa sr85¢1, -dry particle aerosol in~ dicated that the ratio of G.I. tract/respiratory system ac-= tivity was 100 immediately after exposure. Administration of aqueous suspensions of the same material by stomach tube indicated that the amount of sr85 deposited in the skeleton was a considerable fraction of that found inthe skeleton following an inhalation exposure, thus emphasizing the impor- tance of the G.I. tract as a portal of entry. The high concentration of activity in the G.I. tract resulted . \ in an initial dose rate to the small intestine many-fold that received by the respiratory system. The integrated dose received by the large intestine over the 28-day period was twice that received by the respiratory system, stomach or small intestine. The total dose received by the skeleton during the experimental period was 10-fold that received by.any other organ, The importance of the aerosol carrier material was em- phasized by the finding that absorption of Sr into the body was 2 to 3 times greater following the liquid aerosol than following the dry particle aerosol. The finding, that the G.I. tract can serve as an important portal of entry for soluble fission product aerosols following an inhalation exposure, implies that an evaluation of the inter-~nal hazard associated with exposure to radioactive fallout must take into account those parameters which influence the trans- port of particles across the G.I. membrane as well as the more commonly considered factors which determine transport across the alveolar tissue. 11 UNCLASSIFIED