Mean arterial pressure and heart rate data during quiet wakefulness and phases of sleep in conscious rat are sampled by a computer at a rate of 100/sec. Average values and variability expressed as standard deviation are computed for each recording session. Mean arterial pressure and heart rate and their variability decrease from quiet wakefulness to synchronized sleep. During desynchronized sleep, mean arterial pressure increases to the level of quiet wakefulness, and is more variable than during synchronized sleep. Heart rate is lower and more uniform during sleep than during quiet wakefulness, and there is no difference between synchronized and desynchronized sleep except that a greater variability occurs during desynchronized sleep. The study shows that characteristic and specific cardiovascular changes accompany the phases of sleep and that a hierarchy of arterial pressure is present during the resting behavior in rat.