To identify the phospholipase mediating the majority of [Arg8]vasopressin (AVP)-induced release of arachidonic acid in A-10 smooth muscle cells, we exploited the specificity inherent in the mechanism-based inhibitor, (E)-6- (bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one (HELSS), which possesses a 1,000-fold selectivity for inhibition of calcium-independent versus calcium-dependent phospholipases A2. Utilizing [3H]arachidonic acid- labeled A-10 smooth muscle cells, one-half of AVP-inducible [3H]arachidonic acid release was inhibited by pretreatment with only 1 μM HELSS and two- thirds of AVP-stimulated [3H]arachidonic acid release was inhibited by 5 μM HELSS. The inhibition of [3H]arachidonic acid release by HELSS was saturable (i.e. no additional inhibition of [3H]arachidonic acid release was present at 10 μM HELSS), specific (i.e. the activities of six intracellular enzymes, as well as the rate of glucose oxidation, were not altered by HELSS treatment), and nontoxic (i.e. HELSS-treated cells excluded trypan blue dye and did not leak intracellular enzymes into the medium). Collectively, these results demonstrate that HELSS blocks AVP-induced arachidonic acid release by specific and irreversible inhibition of calcium-independent phospholipase A2 and underscore the importance of calcium-independent phospholipase A2 in agonist-induced arachidonic acid release in at least some cell types.