Nitric oxide (NO) is a recognized mediator of inflammation in diseases affecting the lower respiratory tract, and has been implicated in the expression of viral upper respiratory tract infections. Here, exhaled nasal and oral NO concentrations and nitrite concentrations in nasal lavage fluids were measured, symptoms were scored and pulmonary function was evaluated before (Day 0) and after (Days 1-8) experimental exposure of 17 adult subjects to respiratory syncytial virus (RSV) type B. After exposure, RSV was recovered from 12 (70%) subjects by culture and/or specific antigen detection. Both subjects with and those without RSV recovery developed increased nasal and throat symptoms after RSV exposure, but none evidenced changes in the three measures of NO concentration. These results do not support the hypothesis of increased NO production during RSV infection and complement earlier studies that reported a lack of change in nasal NO concentration during experimental influenza and rhinovirus infections.