The left kidneys of Wistar rats were irradiated with a single dose of 20 Gy percutaneously and the local tissue partial oxygen saturation was measured with the multi wire surface electrode (Kessler and Lübbers). The rats were sacrificed at various intervals post irradiation and the pO2-measurements were compared with control values from the unirradiated contralateral kidneys in each animal. Beforehand "normal histograms" were obtained in unirradiated rats. 24 hours post irradiation hyperemia and local perfusion changes were observed, which nearly turned to normal in the course of four weeks. The histograms after 16 weeks proved persisting damage. After a short delay the nonirradiated right kidneys showed similar significant injuries. Further studies are required to clarify these alterations. The multi wire surface electrode render it possible to conceive early functional disturbances in organ microcirculation after high energy irradiation.