REACTIVE oxygen intermediates modulate skeletal muscle contraction^1,2, but little is known about the role of nitric oxide (NO). Here we show that rat skeletal muscle expresses neuronal-type NO synthase and that activity varies among several respiratory and limb muscles. Immunohistochemistry showed prominent staining of type II (fast) fibre cell membranes with antibodies against neuronal-type NO synthase. NO synthase activity in muscles correlated with type II fibre density. Resting diaphragm muscle produced detectable NO_x, but no reactive oxygen intermediates. In contrast, actively contracting muscle generated increased levels of reactive oxygen intermediates. Contractile function was augmented by blockers of NO synthase, extracellular NO chela-tion, and guanylyl cyclase inhibition; it was depressed by NO donors and by increased levels of cyclic GMP. Force–frequency plots of different muscles showed an inverse correlation between NO synthase activity and force development. Our results support two physiological functions of NO in skeletal muscle. The first is to promote relaxation through the cGMP pathway^3,4. The second is to modulate increases in contraction that are dependent on reactive oxygen intermediates and which are thought to occur through reactions with regulatory thiols on the sarcoplasmic reticulum^5,6.