The mechanism by which acetaminophen causes analgesia and antipyretic effect is not entirely clear. Recently a new COX isoenzyme was identified and termed COX 3. In humans this isoenzyme is most abundant in cerebral cortex and heart. Inhibition of COX-3 could represent a primary central mechanism by which acetaminophen decrease pain and possibly fever (10). An effect on of acetaminophen on presynaptic 5-HT(2) receptors in the hypocampus has been demonstrated (11) suggesting again that the primary effect of acetaminophen is in the CNS.