Pretreatment with the opiate antagonist naloxone, at 1.25-5 mg/kg, increased the intensity of methamphetamine stereotypy, had no effect (over a range of 0.3125-5 mg/kg) on apomorphine stereotypy, and antagonized haloperidol catalepsy in rats at 1.25-5 mg/kg. It is suggested that naloxone, by blocking the opiate receptors located on the nigro-striatal and mesolimbic dopamine (DA) nerve terminals, releases the DA systems from endogenous inhibition, presumably caused by endogenous opiate systems, and thereby potentiates methamphetamine stereotypy and antagonizes haloperidol catalepsy. However, the possibility that naloxone might have affected methamphetamine stereotypy and haloperidol catalepsy by modulating the activity of the central noradrenergic and GABAergic systems, which are reported to influence dopaminergically mediated behaviours, also needs to be considered.