The effects of a change from a standard photoperiod (12:12 LD, 45 fc) to that of constant low level illumination (LL, 5 fc) on electroencephalographic activity (EEG), photic seizures and urine cortisol excretion were investigated in the baboon Papio papio. Unlike the normal circadian cycle, in which maximal urine cortisol excretion occurred at 08.00 h and minimal at 20.00 h, rhythms in cortisol excretion were free running under constant light, shifting an average of 2 h/week toward an earlier time of day. The rhythms in seizure susceptibility and EEG spectral power, which were found to be roughly in phase with urine cortisol under conditions of 12:12 LD, were also altered. Average sensitivity to seizures was found to be increased throughout the 24h cycle, although no seizure responses were of the maximal severity obtained in 12:12 LD. A loss of rhythmicity and an overall decrease in total spectral power over the 1-60 c/sec range were observed in background EEG in the LL environment. An 'instability' in relative power within higher (15-30 c/sec) bands over time was also noted. These data suggesting loss of synchronizing cues may lead to disruption of background EEG and exacerbation of epileptic seizures.