Extracellular ATP can p r o d u c e various effects acting via P,-purinoceptors. ATP i s rapidly b r o k e n down b y ecto-ATPase a n d other ecto-enzymes that limit its effect. Further, adenosine, a metabolite of ATP breakdown, can p r o d u c e its own effect acting via P,-purinoceptors, sometimes masking the effects of ATP. An inhibitor o f ATP degradation would be a useful pharmacological tool to discriminate between effects of ATP a n d its metabolites, as well as to potentiate its actions. Diverse c o m p o u n d s that have b e e n claimed to b e inhibitors o f ATP-metabolising ectoenzymes are evaluated, but specific a n d selective Ca2+/Mg2+-dependent ecto-ATPase inhibitors still appear to b e lacking. o 1994 WiIey-Liss, Inc.