This study compared the acute cardiopulmonary and metabolic effects of four high-intensity interval training (HIIT) protocols using varying intensities during the work and recovery periods. Eleven participants (5 males, 6 females) performed four, 20- minute HIIT protocols at a 1:1 "work:recovery" ratio on a cycle ergometer in random order. The work:recovery relative intensities, based upon previously determined maximum work rates, were: 80%:0%, 80%:50%, 100%:0%, and 100%:50%. Oxygen uptake, heart rate, blood lactate, and rating of perceived exertion were measured. Data were analyzed using a two-way, repeated measures ANOV A (p ≤ 0.05). Oxygen uptake and heart rate were expressed as a percentage of the peak values established during a prior graded exercise test. There were clear differences in physiological response between protocols. The 80:50 and 100:0 may produce the best combination of effects. The 100:50 produced the greatest physiological response, however, it may not be practical for the general population.