The duration and degree of reversible cortical ischemia was evaluated by monitoring the direct cortical response (DCR) and cortical blood flow as determined by a thermal diffusion flow probe (CBFp) in a small segment of cortex. Global ischemia was induced in 90 lightly anesthetized cats during recording. The thresholds for attenuation and abolition of the DCR were 23.7 +/- 6.9 (SD) and 8.7 +/- 3.4 ml/100 g/minute, respectively. With the abrupt drop of CBFp to 5 ml/100 g/minute or below, the DCR was obliterated in 2 to 6 minutes. Recovery of the DCR was likely to occur if it had been absent for only 10 minutes, but after 40 minutes without a DCR no animal recovered DCR, even though the animals were monitored for 6 hours. Correlation of CBFp with time of ischemia was done by analyzing the CBFp curve minute by minute and computing the mean CBFp during the ischemic insult. This was then compared to recovery of DCR. By plotting time (T) in minutes, CBFp in ml/100 g/minute, and recovery of DCR, T was identified for levels of CBFp when DCR would be unlikely to recover. The time limits of cortical ischemia are described for blood flows from 0 to 20 ml/100 g/minute. The physiological recovery of the cerebral cortex from ischemia is a function of the depth and duration of the ischemic episode.