The high frequency of recurrent respiratory infections following“overtraining”; in competitive athletes prompted this study to examine the effects of severe exercise on the mucosal immune response. A previous study from this laboratory demonstrated decreased salivary IgA levels after a 50‐km national Nordic ski competition. To further elucidate the specific role of exercise rather than environmental conditions(e.g., cold), we studied competitive bicyclists under controlled laboratory conditions. Eight male bicyclists exercised at 70‐75 percent of VO2max for two hours, and parotid saliva was collected immediately before and after, and 1,24 and 48 hours after exercise. Both salivary IgA and IgM levels decreased immediately after exercise (63 and 44 percent decreases, respectively), remained low for one hour, and returned to pre‐exercise levels by 24 hours after exercise. Salivary IgG, serum IgA, IgG, and IgM levels, and serum antibodies to specific antigens were not changed after exercise, indicating a specific effect on secretory immunoglobulins. These results suggest that severe exercise may be a form of stress associated with changes in mucosal immunity.