Serine racemase is a glial and neuronal enzyme that reversibly converts l -serine to d -serine, an endogenous co-agonist of N -methyl-d -aspartate receptor type glutamate receptors (NMDARs). Here we present methods to recombinantly express and purify serine racemase in bacteria and two complementary ways to determine d -serine levels in unknown samples. Furthermore, a detailed protocol of serine racemase activity assays is described that can be used to screen for activators and inhibitors in vitro.