To study the effect of transient, apnea-induced hypoxemia on electrocortical activity, five patients with severe obstructive sleep apnea syndrome (OSAS) were investigated during nocturnal sleep. Polysomnographic and simultaneous digitized electro encephalographic (EEG) recordings for topographic and compressed spectral array analysis were made. The EEG recordings were timed exactly to respiratory events. During nonrapid eye movement (NREM) apnea, delta band amplitude increased, starting on average 13 seconds after the apnea onset. Average differences were 268% between initial and maximal values and 202% between initial and final values. In contrast, significant increases in delta amplitudes between the onset and end of REM apneas did not occur, although some caused deep oxygen desaturations. Changes in delta activity were not correlated to NREM apnea duration or degree of desaturation. These results indicate that the increased delta activity during NREM apneas may not be caused by arterial hypoxemia. It could instead be due to either an arousal mechanism, since arousals may be preceded by slow waves in EEG, or to a breakthrough of slow-wave-sleep activity. The sleep disturbance in severe OSAS may create such a propensity for slow-wave sleep that stages pass much more rapidly than in normal persons.