A single dose (17.5 mg/kg i.p.) of methamphetamine was administered to iprindole-treated (10 mg/kg i.p.) rats. Forebrain concentrations of methamphetamine and amphetamine were significantly increased in iprindole-treated rats 1 and 6 hr after injection; in contrast to rats pretreated with saline, both amines were also detected after 18 hr. Three and 7 days after injection, significant decreases were seen in tryptophan hydroxylase (TPH) activity and serotonin concentrations in the cerebral cortex, neostriatum and hypothalamus. Hypothalamic TPH activity had recovered by 14 days. Neostriatal tyrosine hydroxylase activity and dopamine concentrations were significantly depressed at all time points examined. Iprindole alone produced a significant increase in cortical TPH activity after 1 day. After 3 days, TPH activity was significantly decreased when compared with control, whereas serotonin and 5-hydroxyindoleacetic acid concentrations were significantly increased. This study demonstrates that persistence of methamphetamine and/or amphetamine at the site of action is important for neurotoxicity.