Allergen-specific nasal challenge is a valid and reliable tool for studying the pathophysiological mechanisms involved in allergic inflammation. Nasal challenge induces an immediate clinical response in allergic subjects and a concomitant appearance of an inflammatory infiltrate. The mucosal inflammation may persist up to 48-72 h after allergen exposure. If the subjects are rechallenged within this period the response is more pronounced: the so-defined priming effect. The aim of the study was to evaluate the effects of the nasal rechallenge, performed at different time intervals: 3 days, 1, 2 and 4 weeks after the first challenge. Forty allergic subjects underwent two nasal challenges: at baseline and after the periods mentioned above (10/group). Symptoms and inflammatory cells (eosinophils and neutrophils recovered by nasal brushing) were assessed. The 3-day-interval group showed a hyperreactive response (priming effect), the 1- and 4-week-interval groups showed a response similar to baseline, and the 2-week-interval group showed a hyporeactive response ('tolerogenic effect'). The last phenomenon may be due to a possible immunologic response similar to that achievable during local specific immunotherapy. The present results further elucidate the kinetics of allergen-driven inflammatory events and highlight the importance of the time period chosen for rechallenge. The latter fact may be of primary importance in clinical trials involving specific challenge.