Oxygen tensions were measured in the cerebral cortex of gerbils subjected to repeated ischemias of short duration. In controls before ischemia, cortex p02 values were found to vary from about 100 to zero torr. These variations did not seem to correlate with depth or topographical location. When this data was assembled into a bar histogram of frequency of particular 02 tensions, the distribution was gaussian shaped with its mean in the 30-35 torr interval. When the animal's brain was made ischemic by bilateral carotid artery occlusion, the p02 dropped rapidly to zero, reaching this value in about 2.5 seconds. The frequency bar histogram showed a shift of p02 values towards low values, 3 x 5 minute ischemias spaced 5 minutes apart shifted the mean down to the 0 to 10 interval. Recovery was sluggish but steady. After 24 hours the histogram was indistinguishable form the pre-ischemic control. The recovery of the p02 was dependent on the duration of ischemia. Very short duration ischemias of a few seconds were followed by immediate recovery of p02 in a few seconds. Longer durations of ischemia (up to one minute), the p02, level recovers promptly with some overshooting. The p02 level rises above the pre-ischemic level by as much as two-fold. With still longer durations of ischemia, (1 minute to 5 minutes), the p02 level rises sluggishly. bilateral carotid artery occlusion, the p02 dropped rapidly to zero, reaching this value in about 2.5 seconds. The frequency bar histogram showed a shift of p02 values towards low values, 3 x 5 minute ischemias spaced 5 minutes apart shifted the mean down to the 0 to 10 interval. Recovery was sluggish but steady. After 24 hours the histogram was indistinguishable from the pre-ischemic control. The recovery of the p02 was dependent on the duration of ischemia. Very short duration ischemias of a few seconds were followed by immediate recovery of p02 in a few seconds. Longer durations of ischemia (up to one minute), the p02 level recovers promptly with some overshooting. The p02 level rises above the pre-ischemic level by as much as two-fold. With still longer durations of ischemia, (1 minute to 5 minutes), the p02 level rises sluggishly.