twenty - four fischer 344 rats were exposed to enriched uranium dioxide ( uo2 ) aerosols to give a mean initial lung burden of 291 + /- 
 89 ( sd ) micrograms . 
 groups of rats were killed at 1 , 7 , 180 , 360 , 540 , and 720 days post - inhalation ( pi ) . 
 their lungs were fixed and inflated . 
 sections cut from all five lung lobes were used to prepare cr-39 neutron - induced 235u fission fragment autoradiographs . a single traverse across a cr-39 autoradiograph of a tissue section , from the left lung of all the rats , was made using a motorized microscopic stage . 
 the traverse was divided into 10 fields . 
 the track counts per field were used to test for homogeneity of track distribution and to assess if there was any tendency for tracks to be related to the peripheral region of the lung . 
 full raster scans across the entire tissue image were made on left lung autoradiographs from two animals killed at each time point to assess the homogeneity of fission fragment track distribution throughout the entire section . 
 there was no evidence of any temporal change in the proportion of tracks associated with the lung periphery . 
 at all time points pi , the track distribution was significantly nonhomogeneous , suggesting a nonuniform pattern of tissue irradiation from the 234u alpha particles . at time points from 180 to 720 days pi , large clusters of macrophages were observed in some of the sections taken from all five lung lobes . the total number of macrophage clusters increased with time pi . 
 these macrophage clusters produced many 235u fission fragment tracks within the cr-39 autoradiographs.(abstract truncated at 250 words)imagesfigure 1.figure 2.figure 3.figure 4 .