The aim of this study was to determine, quantitatively, the production of nitric oxide (NO) in the nose and nasopharynx. Subjects were instructed to perform a Valsalva manoeuvre with their mouth open as gas was aspirated from a closely fitting nasal CPAP mask by a chemiluminescence analyser (Sievers 270B, Sievers Instrument Corp. Boulder, Colorado, U.S.A.). Room air was free to flow in through the mouth and out through the nose and hence to the analyser. The manoeuvre was continued until a smooth plateau of at least 20 seconds in duration was achieved on a chart recorder. The mean plateau concentrations were 176 (+/- 39.6) parts per billion (ppb) for males and 135.8 (+/- 24.4) ppb for females. The mean male production of NO was 15.8 nanomol/min which was significantly different from that of females of 12.5 nanomol/min (Mann-Whitney U Test; P < 0.01). By measuring the concentration of NO in gas aspirated from the nose during Valsalva manoeuvre, we excluded the respiratory tract below the glottis from our sampling and as such results represent the portion of NO produced in the nose and nasopharynx. These findings suggest that nasally produced NO is produced in sufficient quantities to act as a continuous pulmonary vasodilator, being inspired preferentially into areas of greatest ventilation, thus perhaps acting to continually match ventilation to perfusion.