WAY-100635 is a piperazine drug and research chemical widely used in scientific studies. It was originally believed to act as a selective 5-HT1A receptor antagonist, but subsequent research showed that it also acts as potent full agonist at the D4 receptor. It is sometimes referred to as a silent antagonist at the former receptor. It is closely related to WAY-100135. In light of its only recently discovered dopaminergic activity, conclusions drawn from studies that employed WAY-100635 as a selective 5-HT1A antagonist may need to be re-evaluated.