A four-probe implantable resonator used to measure dynamic responses in the rat brain to changes in inhaled oxygen levels. The resonator was inserted at the following coordinates: AP, 0 mm; ML, 2.0 mm and 3.5 mm to the left and right of midline, DV, 5.5 mm. Oxygen was delivered through a nose cone to the rat. Plots from left to right: brain pO2 during change from 30% O2 to air, from air to 15% O2, and from air to carbogen.