Antihistamines are the efficacious drugs to be used for the symptomatic relief of allergic diseases. The safety issue of antihistamines is of central importance because of their widespread use in current medical practice. Positron emission tomography (PET) was used to better understand the pharmacological effects of antihistamines on the central nervous system. The H1 receptor occupancy was examined in young male volunteers with [11C]-doxepin after the oral or intravenous administration of antihistamines. In other studies, the cognitive performance was also measured tachistoscopically before and after taking antihistamines. The H1 receptor occupancy in the human cortex caused by antihistamines is significantly correlated with the reported values of incidence of sleepiness in clinical trials, and the occupancy is well proportional to the impaired cognitive performance. To understand the brain mechanism of antihistamine-induced "sleepiness and impaired cognition", the regional cerebral blood flow (rCBF) during the task was measured using 3D-PET and H2(15)O before and after administration of d-chlorpheniramine. After its administration, the rCBF was significantly decreased on the bilateral middle temporal gyrus, midbrain and anterior cingulate. These findings suggest that H1 receptor blockade would be affected on the activity of the attention and cognitive system in the brain.