Allergic rhinitis and asthma share common epidemiological features and inflammatory processes. The aim of the present study was to document the influence of natural allergen exposure in exhaled NO (eNO) and in spirometric parameters of patients with seasonal allergic rhinitis(SAR) and to investigate the differences among subjects with positive versus negative bronchial provocation to metacholine(BPMch).Twenty-six non-smoking patients (13F/13M; mean age 28.4ys) with a documented history of SAR, 15 healthy, non-atopic(6F/9M; mean age 37.1ys) and 6 non-symptomatic atopic subjects (3F/3M; mean age 36.5ys) were studied. At the first visit during pollen season each subject filled symptom-score card, underwent eNO and nasal NO (nNO) measurements and spirometry. BPMch was performed within the next 10 days. At the second visit out of pollen season, all measurements but BPMch were repeated. Control subjects underwent eNO and nNO measurements.eNO was significantly increased during pollen season in BPMch positive vs BPMch negative(46.22±32.60 vs 17.81±12.67, p=0.014) and vs non-atopic controls(11.40±5.84, p<0.001) as well as atopic controls(13.56±5.34, p=0.001). No difference was detected out of pollen season in both patients' groups. nNO values were increased only in BPMch(+) group compared to both control groups in pollen season (vs non-atopics p=0.002, vs atopics p=0.002) and only vs non-atopics out of season, p=0.004. Regression analysis has shown that the difference in FEF25-75 values (off season-in season) is a predictor of positive BPMch .eNO is markedly increased in BPMch patients with allergic rhinitis while mid-expiratory flow may represent an early marker of lower airway involvement in respiratory allergy.