Oxygen availability (aO2) was measured polarographically in bone, bone marrow, and brains of cats. Immediately after electrodes had been implanted, the mean bone marrow oxygen was erratic. It reached a maximum around the 12th postoperative day, then decreased to a stable level within 2–3 months. "Slow waves" with a frequency slower than in brain (3–8 vs. 8–14 cycles/min) were present in bone marrow but not in compact bone. Under physiological conditions, their frequency and pattern were relatively constant, except during estrus, when they were disrupted in bone marrow, but not in brain. In general, oxygen and nitrogen affected bone marrow aO2 more than brain, but CO2 affected brain aO2 more than bone marrow. Responses of compact bone to these gases were sluggish. Vasoconstrictor drugs decreased mean bone marrow aO2 with variable effects on slow waves. Some vasodilators increased bone marrow aO2. Withdrawal of amounts of blood sufficient to induce erythropoiesis had little effect if any on bone marrow aO2. X-radiation altered the pattern of slow waves.