PPADS (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid) is a selective purinergic P2X antagonist. It is able to block contractions of rabbit vas deferens induced by ATP or ñ,ò,methylene-ATP. It appears to be relatively selective for P2X receptors, having no appreciable activity at ñ<sub>1</sub> adrenergic, muscarinic M<sub>2</sub> and M<sub>3</sub>, histamine H<sub>1</sub>, and adenosine A<sub>1</sub> receptors.