Mast cells occurred along the internal cerebral veins in the brain of 55 female and 75 male Syrian hamsters, 35 to 844 days old. Observations on serial parasagittal sections stained with toluidin blue indicated that the largest aggregation of mast cells was in the arachnoid layer between the velum interpositum near the origin of the internal cerebral veins. Mast cells were aligned along short segments of thalamic veins usually near their entrances into the internal cerebral vein or along capillaries in the thalamus of 28 of 45 hamsters, 500 to 844 days old, but were absent in the thalamus of the 17 less than 200 days old. Mast cells and plasmacytes were present in the choroid plexuses of the lateral and third ventricles in 29 of 41, 521 to 844 days old, but were not found in the choroid plexuses of 42 hamsters, 35 to 381 days old, Plasmacytes but no mast cells occurred in the subfornical organ of 18 of 26, 500 to 775 days old and in only 1 of 13 less than a year old. Hibernation, binding of biogenic amines, and vasodilation are topics of discussion on the significance of heparin and histamine in mast cells.