The effect of intracarotid and intravenous administration of papaverine on local tissue PO2 and microflow in the cat's brain surface was studied. Local tissue PO2 was measured with a multiwire surface electrode polarographically, and microflow by local hydrogen clearance method. The intracarotid infusions were made for 1, 2 and 5 min with doses of 0.1, 0.2 and 0.5 mg/kg/min papaverine, and the intravenous ones for 5 min with doses of 0.2, 0.5 and 1 mg/kg/min. The continuous intracarotid infusions showed that papaverine in the doses used distinctly increased local tissue PO2 and microcirculation of the brain surface. With the doses applied, systemic arterial pressure (SAP) changed little. It slightly decreased only during the 5 min infusions containing 0.5 mg/kg/min. The duration of the effect increased with increases in the duration of the infusion and of the dose. The maximum duration was observed with 5 min infusions and lasted for 10--15 min after drug administration was discontinued. During the i.v. infusions, tissue PO2 and microflow rose less than with intracarotid ones. No redistribution of capillary flow was observed.