This study was to determine the relative effectiveness of four different cardio-respiratory conditioning programs. Two-hundred and twenty-seven healthy male college students participated. All subjects practiced similar programs of general circuit weight training exercises, in addition to the cardiorespiratory exercise regimens. The subjects were divided into four groups. One group exercised according to a simple progressive resistance procedure; which called for a stepwise increase of distance from 440 yards to 880 yards, to 1320 yards, to one mile. The increases were made as soon as the duration was equal to projected lap times for a six minute mile. Another group performed a Fartleck method of varying degrees of intensity and duration for times of 1 min. 10 sec., 2 min. 30 sec., 4 min., and 6 min. One group followed an interval program in which intensities were alternated for equal periods of 60 seconds for 1 min. 10 sec., 2 min. 30 sec., 4 min., and 6 min. The final group ran only two practice runs and the two test runs. This group performed three additional "warm-up" weight lifting exercises designed to stress the cardio-respiratory system. The training period lasted ten weeks. The results showed significant improvement for the first three groups when compaired to the last group. The group which followed the Fartleck protocol improved the most. A subgroup breakdown, of all four groups, based on initial scores, into fast 1/3, middle 1/3, and slow 1/3 was also considered. The subgroup analysis showed the best results of the F 1/3 by the Fartleck method, for the M 1/3 about equal by the Fartleck and Interval techniques, and for the S 1/3 by Progressive Resistance procedure.