We present a new methodology for generating a stepwise concentration gradient in a series of microdroplets by using monolithic micro valves that act as "faucets" in micrometer-scale. A distinct concentration gradient of a substrate was generated for the determination of the kinetic parameters of two different enzymes using only 10 picoliter-scale droplets. With a single experiment on a chip, we obtained K(M) and k(cat) values of matrix metalloproteinase 2 (MMP-2) and matrix metalloproteinase 9 (MMP-9), and compared the catalytic competence of the two enzymes. The present system and method are highly suitable for applications where the reagents or samples are limited and precious.