Endogenously produced nitric oxide (NO) has numerous physiological roles. It is generated by three enzymes: eNOS, nNOS and iNOS. NO produced in endothelia by eNOS regulates blood vessel dilation by targeting vascular smooth muscle cells; NO produced in neurons by nNOS regulates synaptic signalling and plasticity in the brain; and NO produced in large quantities by iNOS in cells of the innate immune system can kill fungal and bacterial pathogens and inhibits viral replication. The Cell Science at a Glance section of JCS is currently featuring a series of contributions covering the various roles of NO. In the second of these, Imran Mungrue and David Bredt tackle NO generated by nNOS (see p. 2627 + poster).