Acetaminophen is the most commonly used drug in children. Inhibition of COX-3 in the brain has been suggested as the primary central mechanism by which acetaminophen decrease pain and possibly fever. However there is very limited data about acetaminophen concentrations in the brain and no such data is available for newborns. The objective of the current study is to describe concentrations of acetaminophen in the fluid around the brain of infants after administration of acetaminophen.