Cyclic nucleotide phosphodiesterases (PDEs) are the enzymes that catalyze the hydrolysis of cAMP and cGMP, thereby restricting the activity of these second messengers in cells. A unique ability to shape gradients of cyclic nucleotides and compartmentalize their signaling implies a high potency and a rapid action of PDEs. However, it has not been demonstrated how fast PDEs can hydrolyze cAMP in a living system. Here we perform a real-time monitoring of PDE2 activity in aldosterone-producing adrenal cells using a recently developed genetically encoded, fluorescent cAMP sensor, which reveals enormously rapid kinetics of cAMP degradation. Activation of PDE2 results in a rapid decrease of intracellular cAMP from high micromolar to the sub-micromolar range within a few seconds. Moreover, the kinetics of atrial natriuretic peptide-stimulated PDE2 activity (measured as decline of cAMP) are much faster than the speed of ACTH and isoprenaline-induced cAMP-synthesis (measured as cAMP accumulation) in the cells, revealing high catalytic activity and fast action of PDEs in regulating cAMP signaling in a physiological system.