The effects of butorphanol and its main metabolites, norbutorphanol and hydroxybutorphanol, on the contents of monoamines and their metabolites in various regions of the rat brain were compared with those of morphine and pentazocine using the HPLC-ECD method. The administrations of morphine and pentazocine increased dopamine turnover in the striatum and hypothalamus in a drug dose-dependent manner. The stimulative effects of butorphanol on the dopamine system were weaker than those of morphine and pentazocine, and there were no dose-dependencies in these effects of butorphanol. Butorphanol, morphine and pentazocine increased 5-HT turnover, but there was no drug dose-dependent effect in the case of butorphanol. These differences for the effects of butorphanol from those of morphine and pentazocine seemed to result from the antagonist-agonist property of butorphanol and from a different manner of interaction with the opioid receptor. The effects of butorphanol on the levels of the norepinephrine system were weak. It was considered that the effects of butorphanol on monoamine turnover were produced by the action of butorphanol itself, because norbutorphanol and hydroxybutorphanol showed little change on the level of monoamines and their metabolites.