We previously showed that acetaminophen administration to rats increases forebrain serotonin levels as a result of the inhibition of liver tryptophan-2,3-dioxygenase (TDO). In this study we determined whether aspirin alone and in combination with acetaminophen could further influence brain serotonin as well as norepinephrine levels and if so whether the status of the liver TDO activity would be altered. The results show that acetaminophen alone increases brain serotonin as well as norepinephrine levels with a concomitant inhibition of liver TDO activity. In contrast, aspirin did not alter the levels of these monoamines but increased serotonin turnover in the brain while acetaminophen decreased the turnover. When combined with acetaminophen, aspirin overrides the reduced serotonin turnover induced by acetaminophen. This report demonstrates the potential of these agents to alter neurotransmitter levels in the brain.