The effects of trimethyltin (TMT) administration on regional concentrations of dopamine (DA), serotonin (5-HT), and their metabolites were determined. Acute administration of 3 or 7 mg/kg TMT (as the chloride) to adult male Long-Evans rats caused alterations in both dopaminergic and serotonergic function in brain at 7 days posttreatment. Dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations were decreased in the nucleus accumbens of rats treated with 7 mg/kg, with a trend occurring with the 3-mg/kg dose group. Conversely, concentrations of DA or DOPAC were not altered in striatum, olfactory tubercle, septum, or amygdala/pyriform cortex. Administration of 3 mg/kg TMT decreased the concentration of serotonin in striatum and nucleus accumbens, and increased the concentration of 5-hydroxyindoleacetic acid (5-HIAA) in hippocampus. The 7-mg/kg dose resulted in increased concentrations of 5-HIAA in striatum, nucleus accumbens, septum, amygdala/pyriform cortex, and hippocampus, and also decreased the concentration of 5-HT only in amygdala/pyriform cortex.