This study aimed to clarify the effects of pinealectomy and sham-surgery on the area postrema (AP) by quantitative histological methods. Male, Wistar rats of normal (NO), sham-operated (SX), and pinealectomized (PX) groups were used in the late dark phase at 7 weeks of age. Consecutive frontal sections including the AP were stained with hematoxylin and eosin, and immunostained using PGP 9.5 for neurons, or GFAP or vimentin for glial cells. Consecutive sections of the AP were separated into five portions starting from the point of the central canal opening to the fourth ventricle in the caudal direction, and used for measurements. Mean cross-sectional areas of capillaries showed a lower value in the SX group than in the other two groups (vs NO, P<0.005; vs PX, P<0.03). In addition, the frequency distributions of the nuclear diameters of nerve cells showed different patterns among the three experimental groups (P<0.01), the frequency of large nuclei being higher in the SX group than in the other two groups. Possible mechanisms of the effects of sham-pinealectomy and pinealectomy and significance of the pineal-AP relation are discussed. The results of this study indicate that stuctural changes in the AP can be induced by intracranial surgery, suggesting certain pineal involvement in these changes.