We had previously found that in animals with moderate nigro-striatal dopamine (DA) lesions (i.e. 45–65% residual neostriatal DA) the mixed D₁/D₂-agonist apomorphine induced ipsiversive rather than the usual contraversive turning found after more radical DA lesions. Since this result promised to provide a behavioral animal model for pre-clinical Parkinson's disease, we hoped to delineate the responsible receptor by challenging with selective D₁- and D₂-agonists. Thus, in the present study, the behavioral effects of the D₁-agonist SKF38393 (5.0 mg/kg) and the D₂-agonist LY171555 (0.5 mg/kg) were tested in drug-naive rats with unilateral 6-hydroxydopamine lesions of the nigro-striatal DA system. This analysis was performed dependent on the degree of the lesion, classified post-mortem with respect to the level of residual DA in the neostriatum: <20%, 20–45%, 45–65%, and >65% (as percentage of the intact hemisphere). The measures of turning, thigmotactic scanning and locomotion did not yield differences between animals treated with the D₁-agonist and vehicle-treated rats. For example, animals with severe lesions (residual DA <20%) showed ipsiversive asymmetries in turning and scanning, which were similar after vehicle or the D₁-agonist, both with respect to degree and time-course. However, the analysis of grooming behavior, which was performed in a subset of animals with moderate lesions yielded differences between vehicle and the D₁-agonist, since the duration of grooming was increased after SKF38393. In contrast to the D₁-agonist, behavioral effects after the D₂-agonist LY17155 were evident in all behavioral measures. The general response to this agonist could be characterized by a rapid decrease of behavioral activity including turning, scanning, locomotion and grooming. Although we failed to find significant behavioral asymmetries with either agonist, a micro-analysis showed evidence for selective effects after the D₂-agonist, since a contraversive asymmetry in turning (and scanning) became apparent between 45 and 60 min after injection in animals with severe lesions (residual DA of about 10% or less), and since there was a weak ipsiversive turning asymmetry in animals with residual DA levels of 45–65%. Such asymmetries were not observed after vehicle or the D₁-agonist. The possible physiological mechanisms of these effects, i.e. DA receptor mechanisms and DA availability, are discussed in the context of results from previous experiments using lesioned or intact animals.