Regional cerebral blood flow (rCBF) was measured using H2(15)O and positron emission tomography (PET) to test the hypothesis that age-related changes in the pattern of rCBF activation would be greater under divided attention conditions than under selective attention conditions. Subjects were 24 right-handed men: 12 young adults (age 21-28 years), and 12 older adults (age 60-77 years). Measurement of rCBF was obtained during performance of three visual search task conditions, each of which involved viewing a series of nine-letter displays and making a two-choice button press response to each display. Analyses of subjects' mean reaction time and error rate confirmed that older adults' search performance was disproportionately impaired when it was necessary to divide attention among the display positions. The rCBF data indicated that attending selectively to a target letter in a known (central) location was not associated with cortical activation for either age group. The requirement to divide attention among the display positions led to rCBF activation in occipitotemporal, occipitoparietal, and prefrontal cortical regions. In the divided-attention condition, rCBF activation in the occipitotemporal pathway was relatively greater for young adults; activation in prefrontal regions was relatively greater for older adults. These differences in rCBF activation were related to search reaction time and suggest that, when attention was divided, young adults' performance relied primarily on letter identification processes, whereas older adults required the recruitment of additional forms of task control.