Percent reduction profiles of cytochromes (cyt.) aa3, b and c were investigated in bloodless guinea pig hippocampal brain slices of 400, 600 and 800 microns in thickness ranging in temperature from 22 to 37 degrees C. The extent of the percent reduction of cytochromes was compared with the generation of orthodromic potentials elicited by the stimulation of the stratum radiatum, and the cessation of the potentials was found to be correlated with the extent of the percent reduction of the cytochromes. In the case of 400 microns slices, they were found to be in normoxia both from the extent of the percent reduction levels of cytochromes and from the generation of orthodromic responses over a range in temperature. In the case of 600 microns slices, those incubated under temperatures of 22 to 32 degrees C were not in hypoxia from the levels of cytochrome reduction and the production of a field potential. However, slices at 37 degrees C were in hypoxia because of cyt. c levels approached those of cyt. b and the orthodromic response was suppressed. In 800 microns slices, those at 22-27 degrees C were in normoxia; however, slices maintained at 32-37 degrees C were in hypoxia because the levels of cyt. c reduction closely approximated those of cyt. b at 32 degrees C whereas those of cyt. aa3, b and c were almost the same as at 37 degrees C. Moreover, the orthodromic field potential was not evoked.