Unilateral injections of the serotonin neurotoxin, 5,7-dihydroxytryptamine (DHT), at various points along the 5-HT pathway to the forebrain produce a turning syndrome associated with alterations of dopamine synthesis in the ipsilateral striatum. Unilateral injections of DHT into the SN produced an ipsilateral increase in striatal dopamine (DA) turnover and contralateral rotation in response to amphetamine or apomorphine. Injection of DHT into the MFB produced an ipsilateral decrease in striatal DA turnover and tyrosine hydroxylase (TOH) activity, and ipsilateral rotation in response to amphetamine or apomorphine. After the injection of DHT into the SN or MFB, there was a significant correlation between the rates of drug-induced rotation, the decrease in cortical 5-HT turnover, and the change in striatal DA turnover, suggesting that the unilateral change in DA turnover (and, presumably, the increased stimulation of DA receptors) is causally linked to turning. Injection of DHT into the zones of the striatum and GP richest in 5-HT terminals produced the same responses as the MFB-lesioned rats: ipsilateral rotation and a decrease in striatal TOH activity. Injection of DHT into the area of the striatum richest in DA terminals failed to produce rotation or a significant change in TOH activity. We suggest that 5-HT neurons from the raphe nuclei exert a tonic inhibition on the nigrostriatal pathway at the level of the SN through direct synapses on DA neurons, whereas their neostriatal terminals have an indirect effect on DA terminals, perhaps via interaction with cholinergic and GABA-ergic neurons.