The goal of this study was to compare the isolated and combined effects of two treatments being used to reduce nasal airflow resistance (NR): an internal nasal mechanical dilator (Nozovent; Prevancure; Sté Pouret, Paris, France) and a topical decongestant, fenoxazoline hydrochloride (Aturgyl; Synthelabo; Le Plessis-Robinson, France). The study was performed in 17 healthy subjects. NR was estimated by active posterior rhinometry at a 0.5 L/s flow under four conditions: in the basal state, with the internal nasal mechanical dilator, after treatment with fenoxazoline hydrochloride, and with both fenoxazoline hydrochloride and the mechanical dilator. The mean NR (+/- SD) decreased from 1.65+/-0.54 cm H2O/L/s in the basal state to 1.02+/-0.27 cm H2O/L/s with the mechanical dilator (p < 0.001), 1.03+/-0.47 cm H2O/L/s with fenoxazoline hydrochloride (p < 0.001), and 0.48+/-0.15 cm H2O/L/s with both the mechanical dilator and fenoxazoline hydrochloride (p < 0.001). The decreases in NR observed after using either the mechanical dilator (deltaNR(N)) or fenoxazoline hydrochloride (deltaNR(A)) were not significantly different. The decrease in NR observed with both (deltaNR(N + A)) was not significantly different from the sum deltaNR(N) + deltaNR(A): 1.16+/-0.53 cm H2O/L/s vs 1.25+/-0.63 cm H2O/L/s, respectively (p > 0.05). deltaNR(N + A) strongly correlated with deltaNR(N) + deltaNR(A): deltaNR(N + A) = 0.80 (deltaNR(N) + deltaNR(A)) + 0.15 (r = 0.96; p < 0.0001). However, the slope of the regression line of deltaNR(N + A) vs deltaNR(N) + deltaNR(A) was significantly lower than unity (p < 0.003). These results demonstrate that, although not totally additive, the effects of using the mechanical dilator and fenoxazoline hydrochloride are cumulative. Further studies that include patients with nasal obstruction would allow us to better evaluate the benefit of a therapy combining both treatments.