1. Serial brain scanning with gammaray scintillation camera and Tc99m per-technetate was performed and the chronologic patterns of serial scintiphotographs were related to the pathology. Regressive accumulation of radioactivity of the focus was observed in arteriovenous anomalies, meningiomas and angioma, whereas the gliomas, metastatic carcinomas, sarcomas, infarctions and abscesses showed a progressive course of isotope uptake. Intracerebral hematomas and cystic tumors the focus remained persistently cold. 2. Chronologic patterns of the uptake ratio were determined digitally and classified into 4 groups. In Group I, the uptake ratio was maximum immediately or shortly after injection and then decreased rapidly. In Group II the uptake ratio showed a gradual increase. In Group III, an initial increase was followed by a decrease, and in Group IV, the uptake ratio remained below 1.0. 3. Xe133 and MAA-I131 scanning data showed that the Group I lesions were rich in vascular bed and had a high blood flow in the focus. Group II and IV lesions were avascular and Group III foci showed a less uniform vascular pattern. 4. It is concluded that the tumor vascularity plays an important role in determining the regressive course of uptake of Tc99m. Another mechanism of isotope uptake, presumably local breakdown or lack of barrier mechanisms, seems to be the main factor in Group II and III uptake patterns.