Rats received intrastriatal or intranigral injections of 6-hydroxydopamine (6-OHDA) on the same side towards which they made most of their turns during a previous test of amphetamine-induced rotational behavior. One week later they were retested for amphetamine-induced rotational behavior and it was found that only approximately half of them increased their rotational behavior towards the lesioned side more than non-lesioned controls. In fact, compared to their pre-operative behavior numerous rats decreased or actually reversed their net turning towards the lesioned side. While the post-lesion rotational behavior of the two groups of rats was clearly different, pre-operative turning was not. Furthermore, the neurochemical effects of the intracerebral 6-OHDA injections were not different in the two groups of rats, either with respect to the magnitude of the resulting dopamine (DA) depletion, or with respect to the compensatory increase in the turnover of DA by surviving DA neurons on the lesioned side. The data are discussed in terms of their lack of support for current notions about the role of nigrostriatal DA in turning, and in terms of their support for a two-population model we have previously proposed. An additional, unrelated, finding from the present work was that bilateral striatal serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels decreased bilaterally one week following unilateral intrastriatal administration of 6-OHDA.