Polarigraphic microelectrodes were used to study the distribution of oxygen tension (pO2) in arterioles (lumen diameters 8-80 μm) and venules (lumen diameters 8-120 μm) in the rat cerebral cortex during acute reductions in blood hemoglobin ([Hb]). Isovolumic hemodilution with 5% albumin solution was performed in steps from an initial [Hb] of 14.1 ± 0.3 g/dl (control) to 9.8 ± 0.3 g/dl (step 1), 6.6 ± 0.4 g/dl (step 2), and 4.6 ± 0.3 g/dl (step 3). Mild anemia (step 1, hematocrit 30%) led to an increase in pO2 in the arterial side of the microcirculatory bed, with virtually no change in pO2 in the venous side. Step 2 (hematocrit 20%) was accompanied by a further insignificant increase in pO2 in arterioles, while there was a significant reduction (on average to 32 mmHg) in venules. Step 3 (hematocrit 13-14%) led to a (statistically insignificant) increase in pO2 in arterioles. pO2 in venules decreased, on average, to 27 mmHg; the proportion of smallest venules with low pO2 values (less than 20 mmHg) increased to 31% (from 3% in controls). In some capillaries, pO2 was 5-10 mmHg, which was an indicator of the presence of hypoxic zones in brain tissues. These zones primarily arose close to the smallest capillary and venous microvessels, with slowed or impaired blood flow.