The effect of hyperbaric oxygen (HBO) at 2.5 atmospheres of absolute pressure (ATA) on postischemic reperfusion injury in striated muscle was studied using two different animal models. One used complete tourniquet ischemia (4 hr) in the rat hindlimb, measuring blood flow in the tibialis anterior muscle by the xenon washout method at 1-1.5 and 4.5-5 hr after the start of reperfusion. The other used 4 hr of pressure-induced ischemia in the fine striated skin muscle, contained within a dorsal skinfold chamber in hamsters. Vital fluorescence microscopy was employed to measure the functional capillary density using fluorescein isothiocyanate-dextran (mw 150,000) as a plasma marker. After 1-1.5 hr of reperfusion following 4 hr of total ischemia, postischemic blood flow was severely depressed, with no significant difference between the HBO-treated and the untreated rats. However, after 4.5-5 hr of reperfusion, blood flow rates in the postischemic muscle in the HBO-treated animals did not differ significantly from those in nonischemic muscle, compared to a persistent, significant depression in the untreated animals. No significant difference was seen in functional capillary density between HBO-treated and untreated hamsters following 1 hr of reperfusion. After 5 hr of reperfusion, capillary density was significantly improved in HBO-treated animals compared to untreated animals. These results suggest that HBO treatment enhances the recovery of blood flow and functional capillary density in postischemic muscle tissue, indicating attenuation of the microvascular dysfunction or damage in the postischemic period.