A-943931 is a potent and selective H4R antagonist with high affinity at both human (Kis = 4.6 nM) and rat (Kis = 3.8 nM) receptors. A-943931 competitively and potently antagonizes rat H4R agonist-mediated responses in [35S]GTPγS binding assays (Kbs = 28 nM) and intracellular calcium mobilization at rat and human receptors (Kbs from 5-10 nM). When tested in vivo in a mouse model of clobenpropit (10 nM, i.d.) induced scratching, A-943931 inhibited the scratching response (IC50 = 26 umoles/kg, i.p.) with significant inhibition (69%) at the highest tested dose (30 umoles/kg, i.p.). A-943931 is a potent and selective H4R antagonist with robust antipruritic activity and is a useful tool for further exploration of the role of H4R receptors in vivo.