This protocol originally proposed to measure brain incorporation of two labeled fatty acids, [11C]arachidonate and [11C]palmitate, as well as rCBF, in young and old normal volunteers, and in patients with Alzheimer disease, Niemann-Pick Type C disease and brain tumors. Eleven patients with Alzheimer disease have been scanned using [11C]arachidonate and [150]H20, compared with 10 volunteers. The current amendment proposes to use only [11C]arachidonate and [150]H20 in 16 additional normal volunteers, and to compare the results between old and young groups and patients with Alzheimer disease. A request to study only 16 additional normal volunteers was approved by the NINDS IRB at the Continuing Review in 1999, and has not changed since then.