Mean oxygen availability (aO2) was measured in the kidney cortex and bone marrow of cats by polarographic technique. O2, CO2, and Apresoline increased the mean aO2, whereas N2, Amobarbital, Pitressin, Adrenalin, Methacholine, venesection, and spinal anesthesia decreased it with varied effects on the cyclic fluctuations in mean aO2. These results suggest that the regulation of kidney aO2 is intermediate between brain and bone marrow oxygen and that kidney hypoxia is not the only factor responsible for erythropoietic stimulation.