The temporal correlation of the secretion of corticosterone (CS) and grotwh hormone (GH) with the sleep-wakefulness cycle in adult male rats was studied by serial blood sampling at 10-min intervals over a 11-hour period from 11.00 to 22.00 h. Cortical EEGs recorded continuously during the blood sampling were scored into wakefulness and sleep, and the amounts of sleep for every 10 min was plotted against the CS and GH values. All 11-hour time series of CS, GH and amount of sleep were found to have three major ultradian rhythms with periodicities of 1.5 h and its multiples. For CS, there was a significantly negative cross-correlation between the amounts of sleep. Although the most prominent CS rhythm was the 1.5-hour period, CS secretion occurring with a 3.0-hour period had such a phase relationship with the 3.0-hour period sleep rhythm that the secretion began in the late stage of the sleep cycle and reached its peak around the time of wakefulness between sleep cycles. In contrast, a GH secretory burst occurred in the early stage of the sleep cycle occurring with a 3.0-hour period, with a definite time lag after the onset of the sleep cycle. The present study demonstrates that three different functions, CS, GH secretions and sleep-wakefulness, have common ultradian rhythms with similar periods, and manifest their rhythms based on their own pacemakers. It is assumed further that each pacemaker is fixed in specific phase relations with others.