Fluid of the parotid gland was collected from seven human subjects during sleep and four subjects during wakefulness. It was continuously weighed during collection on an electronic balance. The weight value was processed on line by a personal computer, and circadian rhythm in connection with the flow rate of saliva was studied. One subject showed no secretion of saliva throughout sleep, but in the other subjects, saliva was secreted, although it was slight. There was no difference between the flow rate in rapid eye movement (REM) sleep and non-REM sleep. Every subject during wakefulness showed a flow rate in a range from a very high value to one almost equal to the value during sleep. The flow rate during wakefulness varied greatly, and there seemed to be no rhythmic pattern to it. With the aid of auditory or visual stimuli etc., the influence of emotional state upon parotid secretion was investigated in 12 subjects. It was found that the flow rate decreased significantly from an intense, passionate state to a relaxed, pleasant state, to a tense or uneasy state and was lowest in a passive, indifferent state, being almost the same as that during sleep. These findings suggest that secretion of parotid saliva depends more on emotional state than circadian rhythm.