In brief: The heart rates of 15 racquetball players were monitored during singles and doubles competition. During a one-hour singles match subjects attained an average of 83% of their maximum heart rate reserve (MHRR) and played above 60% of their MHRR for 56 continuous minutes. During doubles competition, the subjects averaged 67% of their MHRR and played above 60% of their MHRR for 29 continuous minutes. The total active playing time above 60% MHRR was 59 minutes in singles and 45 minutes in doubles. Doubles is less strenuous than singles competition in racquetball, but both forms of exercise meet the criteria for exercise intensity established by the American College of Sports Medicine for developing and maintaining cardiorespiratory fitness.