Racemic trans-10,11-dihydroxy-5,6,6a,7,8,12b- hexahydrobenzo[a]phenanthridine (2, dihydrexidine) was shown previously to be the first bioavailable full efficacy agonist at the D1 dopamine receptor. In addition to its full D1 agonist properties, 2 also is a good ligand for D2-like dopamine receptors. The profound anti-Parkinsonian actions of this compound make determination of its enantioselectivity at both D1 and D2 receptors of particular importance. To accomplish this, the enantiomers were resolved by preparation of diastereomeric (R)-O-methylmandelic acid amides of racemic trans-10,11-dimethoxy-5,6,6a,7,8,12b- hexahydrobenzo[a]phenanthridine 4 that were then separated by centrifugal chromatography. An X-ray analysis of the (-)-N-(R)-O-methylmandel diastereoamide revealed the absolute configuration to be 6aS,12bR. Removal of the chiral auxiliary and O,O-deprotection afforded enantiomeric amines that were then tested for biological activity. In striatal membranes, the (6aR,12bS)-(+)-enantiomer 2 had about twice the affinity of the racemate and 25-fold greater affinity than the (-)-enantiomer at the D1 receptor labeled by [3H]SCH23390 (K0.5s of 5.6, 11.6, and 149 nM, respectively). Similarly, the (+)-enantiomer 2 had about twice the affinity of the racemate for human D1 receptors expressed in transfected Ltk- cells. Functionally, the (+)-enantiomer of 2 was a full agonist, with an EC50 of 51 nM in activating striatal dopamine-sensitive adenylate cyclase versus 2.15 microM for the (-)-enantiomer. With respect to D2-like receptors, (+)-2 had a K0.5 of 87.7 nM in competing with [3H]spiperone at D2 binding sites in rat striatal membranes versus about 1 microM for the (-)-enantiomer. Together, these data demonstrate that both the D1 and D2 activities of dihydrexidine reside principally in the (6aR,12bS)-(+)-enantiomer. The results are discussed in the context of structure-activity relationships and conceptual models of the D1 receptor.