Parkinson's disease (PD) is characterized by nigrostriatal dopamine (DA) neuron degeneration, with L-Dopa (most effective but causing dyskinesia) and D2-selective agonists (cotherapy) as main treatments. Previous D1 agonists like SKF-38393 lacked efficacy due to low intrinsic activity, while full D1 agonists dihydrexidine and A-77636 showed acute efficacy but tolerance upon repeated administration. A new class of DA agonists was developed, leading to compound 5 [(−)-(5aR,11bS)-4,5,5a,6,7,11b-hexahydro-2-propyl-3-thia-5-azacyclopent-lena[ c]phenanthrene-9,10-diol, A-86929], a novel D1-selective agonist. Compound 5 exhibited high affinity (Ki = 49 nM) and full intrinsic activity (125% relative to DA) for the human D1 receptor, with good selectivity over D2 (8-fold in binding, ~300-fold in adenylate cyclase). In the 6-hydroxydopamine-lesioned rat rotation model, 5 produced robust contralateral rotation (subcutaneous ED50 = 0.04 μmol/kg) and maintained behavioral efficacy upon once-daily administration for 10 consecutive days. Due to the lability of 5's catechol group (50% degradation at 60 °C for 6 days), it was converted to the diacetyl prodrug 6 (ABT-431), which showed improved solid-state stability (no detectable degradation under the same conditions) and rapid conversion to 5 in rat blood and liver/jejunal homogenates (half-life < 1 min). Prodrug 6 had similar in vivo efficacy (subcutaneous ED50 = 0.02 μmol/kg) and maintained efficacy upon three-times-daily administration for 10 days. Both compounds were orally active (ED50 = 5.5-8 μmol/kg). Thus, compound 5 is a potent and selective D1 agonist that maintains behavioral efficacy upon repeated administration, and its prodrug 6 addresses stability issues, representing promising preclinical candidates for PD treatment.