There are at least three distinct nitric oxide (NO) synthase genes. Brain and endothelial NO synthases are constitutively synthesized, while NO synthase is inducible by cytokines in macrophages. We have utilized a backcross of (C57BL/6J x Mus spretus) x C57BL/6J to map the inducible NO synthase (Nos2). We report the chromosomal mapping of Nos2 to mouse chromosome 11, 3.3 cM proximal to Scya2.