The purpose of this work is to investigate the role of the frontal lobe, and its connections for cognition in health and in neuropsychiatric diseases, particularly schizophrenia. Regional cerebral blood flow (rCBF) will be measured with oxygen-15 water positron emission tomography (PET) while subjects perform a variety of neuropsychological tests. Some of these tests involve cognitive operations that are posited to depend upon the frontal cortex, such as the use of working memory for abstract reasoning and problem solving, formation and maintenance of conceptual sets, set shifting, sequencing, and delayed response; others control for nonspecific sensory and motor aspects of these measurements or are contrast conditions posited to depend on other brain regions. Interactions between regional activation, cognitive behavior, and neuropharmacology will be assessed by measuring cognitively-related rCBF during treatment with drugs that may affect frontal lobe physiology. Hypotheses about genetic determinants of these relationships will also be tested by comparing rCBF measures across individuals harboring different genotypes (detemined through protocol 95-M-0150).