The temporal effects of 5.0 Gy of radiation on healing subcutaneous microvasculature were studied using a window chamber in the dorsal flap of the Fischer-344 rat. Microvascular function was assessed by morphometric and dynamic flow measurements which were made prior to and at 24 and 72 h after exposure. A comparison was made between chamber preps that were 3 and 14 days postsurgery. The hypothesis of the study was that the older preparation would be more refractory to damage by radiation. Both unirradiated preparations showed an increase in capillary numbers over the period of observation, while irradiated preps had a reduction, especially in vessels less than 50 microns in diameter. Red cells velocities increased by 20-100% in those vessels which survived the radiation exposure, indicating that tissue oxygen tensions might be preserved in spite of a loss of vasculature. These results explain the need for both morphologic and dynamic flow measurements when assessing the effect of therapeutic intervention on microcirculatory function. Further studies are underway to identify a fully mature capillary bed in this model, since it is apparent that capillary growth is continuing in the 14-day preparation.