After unilateral lesions of the mesotelencephalic dopamine (DA) system, the administration of DA receptor agonists results in circling. This response is believed to reflect an asymmetry in mesotelencephalic DA activity. Moreover, drug-induced circling is thought to be directed away from the projection of higher dopaminergic activity. Recently, it has been reported that circling can be established and maintained using operant procedures in surgically intact and drug naive rats. The phenomenon of conditioned circling has been associated with an asymmetrical change in DA metabolism within the striatum and nucleus accumbens. The present series of experiments was designed to characterize further the involvement of mesotelencephalic DA in conditioned circling.Rats trained to circle for water according to a continuous schedule of reinforcement did not exhibit increased DA metabolism within either the striatum or the nucleus accumbens (Experiment I). However, a bilateral augmentation was observed when rates of responding were increased by implementing an intermittent schedule of reinforcement (Experiment II). Concurrent increases in the biosynthesis of DA, as estimated by accumulation of DOPA following the administration of a DOPA decarboxylase inhibitor, were not observed (Experiment III). Experiments IVa and IVb examined the extent to which inherent directional biases, which play a role in determining the magnitude and direction of drug-induced circling, influenced the acquisition and performance of the conditioned circling response. No effects were evident. Moreover, a symmetrical, bilateral enhancement in DA metabolism was observed in the striatum, irrespective of directional preferences. While conditioned circling can be established and maintained by reinforcing the response with food, food itself influenced DA metabolism and therefore precluded the detection of changes in DA metabolism specific to the circling response. Specifically, striatal and accumbens DA metabolism was augmented to a similar extent in animals given matched amounts of non-contingently presented food (Experiment V). Concentrations of DA, DOPAC and homovanillic acid (HVA) were found to be differentially distributed throughout the striatum (Experiment Via), suggesting a possible chemical basis for the heterogeneity of striatal DAergic functions. Changes in striatal DA metabolism associated with conditioned circling were observed only within localized regions of the anterior striatum (Experiment VIb). All changes noted were, however, bilateral in nature. Finally, unilateral lesions of the mesotelencephalic DA projection, following the establishment of the conditioned circling response, disrupted responding, irrespective of the relative locus of the lesion (i.e. ipsilateral or contralateral to the direction of turning) (Experiment VII). However, the extent of the behavioral deficit was more severe following contralaterally placed lesions.It is concluded that circling, established and maintained by positive reinforcement, is subserved by a bilateral augmentation in DA metabolism within the nucleus accumbens and discrete regions of the striatum. However, lesion studies indicate an asymmetrical involvement of the ipsilateral and contralateral projections in this response.