Using rat mast cells permeabilized with streptolysin O we show that release of arachidonate generally occurs under similar but not identical conditions to those that cause exocytosis of beta-N-acetylglucosaminidase (hexosaminidase). Thus, hexosaminidase secretion and arachidonate release both require provision of Ca2+ together with a guanine nucleotide but exocytosis occurs at lower concentrations of both effectors. The kinetics of both processes are similar, with a delay in onset only when ATP is present. Arachidonate release occurs largely from a pool of arachidonyl phosphatidylcholine which appears to represent less than 1% of the total phosphatidylcholine of the cells. Despite the general similarity of the conditions causing exocytosis and arachidonate release, our results show that under some circumstances it is possible to obtain exocytosis without measurable release of arachidonate and that therefore phospholipase A2 activation is not an essential precursor of secretion.