Seventy-three rats were exposed to an aerosol of enriched uranium dioxide (UO2), giving initial lung burdens of 26 to 447 micrograms at 6 days post-inhalation (PI). At 7 days PI 35 of these rats were further exposed to thermalised neutrons at a fluence of 1 x 10(12) neutrons cm-2. There was no significant difference between the two groups in the clearance rate of the UO2 particles from the lung, up to 590 days PI. The particles cleared relatively slowly over this period with a retention half-time in the lung of 160 to 176 days. Transmission electron microscope (TEM) studies of tissue from the alveolar region at 8 days PI showed that inhalation of UO2 particles significantly increased the sizes of macrophage and type II cells, and the number of macrophage and type I cells. There was also a significant increase in the size of lysosomal granules within the macrophages after exposure to the UO2 particles. The exposure to UO2, neutrons and 235U fission fragments had no significant effect on any of the cells above that observed in the animals exposed to UO2 alone. Additional rats were exposed to the same neutron fluence without prior UO2 inhalation. The alveolar cells of neutron-only exposed rats were, in size and number, typically no different from those in the completely unexposed control rats.