Taste perception depends not only on the chemical and physical properties of tastants, but may also depend on the physiological and psychological conditions of those who do the tasting. In this study, the effects of mood state on taste sensitivity was evaluated in humans who were exposed to conditions of mental or physical fatigue and tension. Taste responses to quinine sulfate (bitter), citric acid (sour) and sucrose (sweet) were tested. The intensity of the taste sensations were recorded by a computerized time-intensity (Tl) on-line system. Subjects performed mental tasks by personal computer or physical tasks by ergometer for 10-40 min. Before and after these sessions, the duration of the after-taste and the intensity of the sensation of taste were recorded by the Tl system, and in addition, psychological mood states were evaluated with POMS (Profile of Mood State). Tl evaluation showed that after the mental tasks, the perceived duration of bitter, sour and sweet taste sensations was shortened relative to the control. Total amount of bitterness, sourness and sweetness was also significantly reduced. Furthermore, the maximum intensity of bitterness was significantly reduced. There were no significant differences in bitterness and sweetness sensations following physical tasks. However, relative to before the physical task, the duration of the after-taste of sourness was significantly shortened by the physical task. After the physical task, the buffering capacity of saliva was significantly increased. Thus mental and physical tasks alter taste perception in different ways; the mechanisms underlying these changes remain to be determined.