Phospholipase (PL) A₂ activity prepared from isolated rat fat pads incubated with sodium orthovanadate (vanadate) was increased in a time-and dose-dependent manner. The increasing effect of vanadate was reduced in the presence of tyrosine kinase inhibitors. Under the inhibition of protein synthesis by cycloheximide, vanadate still showed a full effect on the increase in PL A₂ activity. Various PL A₂ inhibitors, such as manoalide, quinacrine and p-bromophenacyl bromide, suppressed the stimulatory release of lipoprotein lipase (LPL) activity from the fat pads by vanadate. Moreover, the vanadate-stimulated release of LPL activity was decreased by the cyclooxygenase and thromboxane synthetase inhibitors, and a thromboxane A₂ receptor antagonist, but was never suppressed by a lipooxygenase inhibitor. The stimulatory release of LPL activity by vanadate was also decreased in the presence of tyrosine kinase inhibitors. These results suggest that vanadate increases PL A₂ activity, and the increase in PL A₂ activity is partly involved in the vanadate-stimulated release of LPL activity with an association to the membrane tyrosine kinase.