A nasal dilatator contains two elastic strips, which provide the dilatator with a spring action. The aimed function of the nasal dilatator is to slightly open the nares and hereby facilitate nasal breathing. The aim of this study was to evaluate the effect of a nasal dilatator by measuring nasal airway resistance during normal breathing and nasal forced expiratory and inspiratory flows and volumes with and without use of the nasal dilatator. Nasal resistance was measured with a whole body plethysmograph; maximal expiratory and inspiratory flow-volume curves were obtained with a pneumotachometer. These measurements were performed in ten healthy volunteers. No significant difference was found between nasal resistance with and without the nasal dilatator. However, values for forced inspiratory volume in 1 s (FIV 1) with the nasal dilatator, proved to be significantly higher (p=0.045,paired t-test) than values obtained without the nasal dilatator; mean improvement was 0.26 L (sd = 0.36). No significant improvement in peak inspiratory flow (PIF) was found, as was the case for the other flows and volumes. It is concluded that the nasal dilatator causes no appreciable improvement of nasal patency during normal breathing. In view of the fact that FIV 1 values increased significantly, we believe that the nasal dilatator prevents collapse of the external nares during forced inspiration. A beneficial effect during exercise when ventilation is increased is however doubtful because in that situation most volunteers switch to oronasal breathing already at submaximal exercise.