Trimethyltin (TMT), an organic metal, has been known to induce behavioral abnormalities including seizures and aggression. We administered TMT to rats, then, behavioral changes as well as the changes of dynorphin and Met-enkephalin mRNA were observed with or without phenobarbital treatment in order to reveal the role of neuropeptides in seizure-generating mechanisms. Met-enkephalin mRNA was significantly increased at the 2nd to 6th day after TMT administration when seizure was frequently observed. Meanwhile, dynorphin mRNA was decreased significantly from the 2nd day to 16th day during aggression score remained high. Phenobarbital abolished not only seizures and aggression, but also the changes of neuropeptide expressions. These results suggest that the changes of dynorphin mRNA are more strongly associated with aggression than seizures, while Met-enkephalin changes correlate more with seizures.