We have realized the direct ionization of samples from wells of microtiter plates under atmospheric conditions for mass spectrometry analysis without any liquid delivery system or any additional interface. The microtiter plate is a commercially available 384-well plate without any modification, working as a container and an emitter for electrostatic spray ionization of analytes. The approach provides high throughput for the large batches of reactions and both the qualitative and quantitative analysis of a single compound or mixture. The limits of detection in small drug molecules, peptides and proteins are similar in comparison with standard direct infusion electrospray ionization. The analysis time per well is only seconds. These analytical merits benefit many microtiter plate-based studies such as combinatorial chemistry and high throughput screening in enzyme assay or drug metabolism. Herein, we illustrate the application in enzyme assay using tyrosine oxidation catalyzed by tyrosinase in the presence or absence of inhibitors. The potential application in drug development is also demonstrated with cytochrome P450-catalyzed metabolic reactions of two drugs in microtiter plates followed with direct ESTASI-MS/MS based characterization of the metabolism products.