We recently observed rapid shallow breathing during recovery from maximal exercise in some normal subjects. We wondered whether this phenomenon is randomly related to level of exercise or is limited to recovery from very high levels of exercise. We monitored ventilation, tidal volume, and respiratory frequency in seven normal subjects during and after exercise. Each subject exercised on several occasions on separate days. At least two of the tests were maximal (i.e., subject terminated). In the other tests exercise was terminated by the experimenter at different fractions of the highest level attained by the subject. There was no systematic difference between breathing pattern during exercise and recovery in tests where final O2 consumption (VO2) was 45-92% of the subjects' highest VO2. By contrast 13 of 19 studies in which final VO2 was 92-100% of highest VO2 were followed by relative rapid shallow breathing. We conclude that rapid shallow breathing during recovery from exercise is a phenomenon that is limited to very high exercise levels. On consideration of the various mechanisms that may be entertained to explain this phenomenon, we believe that development of pulmonary congestion-interstitial edema at very high levels of exercise is the most consistent with our findings.