Evidence from preclinical and human studies demonstrate that arginine vasopressin and vasopressin receptor 1a (V1a-R) play a crucial role in the pathophysiology of psychiatric disorders such as anxiety, depression and post-traumatic stress disorder (PTSD). The lack of selective, brain-penetrant and orally active V1a receptor antagonists has hampered the progress in our understanding of the precise/complete pharmacological role of V1a-R in central nervous system disorders and the development of novel treatments for these disorders. The optimization of promising V1a-R antagonist lead compounds and testing in known and relevant in vivo animal models will be a significant step forward in our understanding of the role of V1a-R in CNS disorders and in the development of novel treatments of anxiety, depression and PTSD. The lack of published data regarding brain-penetration and off-target selectivity, along with intellectual property barriers of the known V1a-R antagonists precludes their potential usefulness as CNS agents. As a result, there is a substantial need to discover centrally active V1a-R antagonists that are useful as in vivo tools to better understand the pharmacological role of V1a-R in CNS disorders with the potential as clinically therapeutic agents. The Scripps Research Institute Molecular Screening Center (SRIMSC), part of the Molecular Libraries Probe Production Centers Network (MLPCN), reports ML389 as a highly potent V1a-R antagonist with an IC50 of 40 nM, and high selectivity (selectivities vs. vasopressin 1b receptor, vasopressin 2 receptor, and the oxytocin receptor of >1250 fold). ML389 was identified by medicinal chemistry optimization of lead compounds. A set of pharmacokinetic analyses show that ML389 has good brain penetration, no significant activity at four human cytochrome P450 subtypes, high binding for human and rodent plasma protein, and an encouraging in vivo pharmacokinetic profile in mice. Broad selectivity screening data against a panel of receptors, transporters, and ion channels suggest that ML389 is generally inactive against a broad array of off targets and does not likely exert unwanted effects. ML389 serves as a novel V1a-R antagonist that can be developed as a therapeutic for the treatment of psychiatric disorders such as anxiety, depression and PTSD.