The regional brain histamine regulation in response to stress was investigated in 12 month old Sprague-Dawley male rats. Air blast exposure (15 min) induced significant (26.5%) elevation in hypothalamic HA level; midbrain and cortical HA concentrations were not affected. Histamine methyltransferase activity was not altered by stress in any of the brain regions investigated. Plasma corticosterone levels of stressed rats were significantly elevated (6.5 fold). Hence, the response of hypothalamic HA to stress is still evident in 12 month old rats.