Presented in part at the Fourth Annual Meeting of the International Society for Magnetic Resonance in Medicine, New York, April 27–May 3, 1996, and at the Second International Conference on Functional Mapping of the Human Brain, Boston, June 17–21, 1996. Received April 11, 1997; revision received Sept. 2, 1997; accepted Sept. 5, 1997. From the Laboratory for Cerebral Blood Flow, Brain Imaging Center, McLean Hospital, Consolidated Department of Psychiatry, Harvard Medical School. Address reprint requests to Dr. Levin, Brain Imaging Center, McLean Hospital, 115 Mill St., Belmont, MA 02178; firstname.lastname@example.org (e-mail). Supported in part by grants DA-00297 (Dr. Levin), DA-09448 (Dr. Renshaw), DA-04059 and DA-00064 (Dr. Mendelson), and DA-00101 (Dr. Mello) from the National Institute on Drug Abuse. The authors thank Anne Smith, RT-R, and Eileen Connolly, RT-R, for technical assistance.