Injections of p-CA or 5,7-DHT into the MRN caused decreases in 5-HT and 5-HIAA levels in the cortex and striatum. They also caused a decrease in 5-HT turnover in the cortex and an increase in striatal DA turnover. The unilaterally injected rats showed contralateral rotation after amphetamine or apomorphine administration. There was a significant correlation among the rate of rotation, the decrease in cortical 5-HT turnover, and the increase in striatal DA turnover. Injection of 5,7-DHT into the SN produced the same biochemical and behavioral changes as did MRN lesions, suggesting that the changes induced by MRN lesions might be due to a direct projection from the raphe to the substantia nigra. Injection of 5,7-DHT into the MFB caused ipsilateral rotation after injection of amphetamine or apomorphine and a decrease in DA turnover in the striatum. There was a significant correlation among the rate of rotation, the decrease in 5-HT turnover in the cortex, and the decrease in striatal DA turnover in the MFB-lesioned rats. These effects might be due to a projection from the DRN to cell bodies in the striatum.