Adenosine 5′-triphosphate (ATP) induced a non-cytotoxic, calcium-dependent release of histamine and prostaglandin D2 from rat serosal mast cells. The effect was tissue and species specific. In particular, tissue mast cells from the guinea pig and were totally unresponsive. The release evoked by the nucleotide was unaffected by antiasthmatic chromones but was inhibited by structurally related flavonoids and by cAMP-active drugs. Secretion was blocked both by the P2Y-purinoceptor antagonist reactive blue 2 and the P2x-antagonist suramin, whereas the P2-agonists 2-methylthio ATP, α, β-and β,α-methylene ATP were inactive. These findings indicate that ATP acts through a novel purinoceptor or binding site which is present on certain types of mast cells.