Twenty-seven firemen were divided into three groups. Group A performed one hour of unsupervised exercise three to four times per week. Group C performed similar but supervised exercise, and group B had no exercise program. After 12 weeks group A showed an average increase of 19% in maximal oxygen uptake (V02max); group C, an average increase of 20%; and group B, an average decrease of 2%. One-minute postalarm heart rates showed a correlation with V02max (p=.15). A serious cardiac arrhythmia was found in the oldest fireman. Careful physical screening followed by an on-the-job exercise program increases aerobic reserve. We suggest that postalarm tachycardia is dampened. This may lessen the risk of heart disease occurring in the postalarm period.