Cromakalim (BRL 34915), known as an airway smooth muscle relaxant, inhibited the releases of mediators in the antigen-induced mast cell activation. It has been suggested that cromakalim, in part, inhibited mediator releases by inhibiting the initial increase of 1,2-diacylglycerol (DAG) produced by the activation of the other phospholipase system which is different from phosphatidylcholine-phospholipase D pathway. The aim of this study is to further examine the inhibitory mechanism of cromakalim on the mediator release in the mast cell activation. Guinea pig lung mast cells were purified by using enzyme digestion and percoll density gradient. In purified mast cells prelabeled with [3H]PIP2, phospholipase C (PLC) activity was assessed by the production of [3H]insitol phosphates. Protein kinase C (PKC) activity was assessed by measuring the protein phosphorylated from mast cells prelabeled with [γ-32P]ATP, and Phospholipase A2 (PLA2) activity by measuring the lyso-phosphatidylcholine produced from mast cell prelabeled with 1-palmitoyl-2-arachidonyl phosphatidyl-[14C]choline. Histamine was assayed by fluorometric analyzer, and leukotrienes by radioimmunoassay. The PLC activity was increased by activation of the passively sensitized mast cells. This increased PLC activity was decreased by cromakalim pretreatment. The PKC activity increased by the activation of the passively sensitized mast cells was decreased by calphostin C, staurosporine and cromakalim, respectively. The PLA2 activity was increased in the activated mast cells. The pretreatment of cromakalim did not significantly decrease PLA2 activity. These data show that cromakalim inhibits histamine release by continuously inhibiting signal transduction processes which is mediated via PLC pathway during mast cell activation, but that cromakalim does not affect PLA2 activity related to leukotriene release.