Nowadays, the neural receptor study has been reported in many research papers. Human studies related with neural receptors have made progress by the technological improvement of positron emission tomography (PET) and development of various radiopharmaceuticals that can specifically bind to neuroreceptors. A previous study by Koepp et al. revealed, in 1998, that the release of neuronal dopamine had been increased in human brain during performing a "goal-directed task" (video game). Given an idea from this previous study, we hypothesized that the release of histamine, another neurotransmitter in the brain, might change under the influence of physiological factors such as stress and circadian rhythm. The purpose of our present study was to evaluate the change of brain histamine release induced by stress and circadian rhythm in the healthy human subjects by using [11C]doxepin and PET method. In results, there was a trend for decreased BP in the condition of calculation task, but the difference was not significant, demonstrating that the histamine release in the brain would not be influenced so much by psychological stress. However, the BP was higher in the morning than in the afternoon. The result indicates that histamine release in human brain would be higher in the afternoon than in the morning.