Viral infection may induce the expression of nitric oxide (NO) synthase, resulting in increased NO formation that has an antiviral effect. NO may be produced by various cells of the upper and lower respiratory tract, and may be detected in the exhaled air. We have studied the levels of exhaled NO in 18 normal subjects during symptomatic upper respiratory tract infections and during recovery 3 weeks later. Exhaled NO was measured using a modified chemiluminescence analyser. At the time of symptoms of upper respiratory tract infection, the peak exhaled NO values were 315 +/- 57 ppb (mean +/- SEM) and decreased to 87 +/- 9 ppb during recovery. Recovery values of exhaled NO were similar to those reported in age-matched normal control subjects (88 +/- 3 ppb, n = 72). These findings suggest that symptomatic upper respiratory tract infections markedly increase the concentration of NO in exhaled air. This may reflect the induction of nitric oxide synthase (NOS) in upper and lower respiratory tract, and may be relevant to viral exacerbations of asthma.