Rabbit vital microscopic bone chambers were used to study acute irradiation effects on the microvasculature in bone tissue in rabbits. This in vivo technique makes it possible to study the temporal development of events in a well-defined tissue compartment. During a 4-hour observation period following 25 Gy single dose of Co60-irradiation, increasing leukocyte adherence to the venular endothelium and subsequent migration into the interstitial space was observed. The capillaries showed reduced and occasional reversal of blood flow. At no time during the entire observation period were there any signs of macromolecular leakage as studied with FITC-dextran 70 and epi-illuminescence.