Chemical dimerizers are powerful tools for non‐invasive manipulation of enzyme activities in intact cells. Here we introduce the first rapidly reversible small‐molecule‐based dimerization system and demonstrate a sufficiently fast switch‐off to determine kinetics of lipid metabolizing enzymes in living cells. We applied this new method to induce and stop phosphatidylinositol 3‐kinase (PI3K) activity, allowing us to quantitatively measure the turnover of phosphatidylinositol 3,4,5‐trisphosphate (PIP3) and its downstream effectors by confocal fluorescence microscopy as well as standard biochemical methods. Little helper: Chemical dimerizers are powerful tools for manipulation of enzyme activities in intact cells. The first rapidly reversible chemical dimerization system is introduced, which permits to determine kinetics of lipid metabolizing enzymes in living cells. This new method was applied to induce and stop phosphatidylinositol 3‐kinase activity, allowing to quantitatively measure 3,4,5‐trisphosphate turnover.