The critical power test provides two measures, critical power (CP) and anaerobic working capacity (AWC). In theory, the CP measurement represents the maximal power output that can be maintained without fatigue, which is regarded as an aerobic measure. AWC is an estimate of work capacity associated with muscle energy reserves adenosine triphosphate (ATP) and phosphocreatine (PCr). High- intensity interval training (HIIT) has been shown to be an effective training method for improving endurance performance, including CP and AWC measures. In addition, creatine (Cr) supplementation has been reported to improve AWC without training; however, Cr had no effect on CP. To date, no one has examined the effect of Cr supplementation during HIIT on CP and AWC. The purpose of this study was to examine the effects of four weeks of HIIT in conjunction with Cr supplementation on CP and AWC in college-aged men. Forty-two recreationally active men (mean ± SD; age 23.62 ± 4.78 yrs; height: 177.48 ± 7.05 cm; weight: 82.23 ± 12.70kg) volunteered to participate in this study. Participants were assigned to one of three groups: Cr (n = 16) 10g Cr + 10g dextrose powder blend; placebo (PL, n = 16) 20g dextrose powder blend; control (CON, n = 10) no treatment. Prior to and following supplementation, each participant performed a continuous maximal oxygen consumption test (o2PEAK) on a cycle ergometer to establish peak power output (PPO). Participants then completed a CP test, consisting of three exercise bouts to exhaustion with the workloads set as a percentage of their PPO to determine CP and AWC. Following initial testing, a 2-week familiarization period of training and supplementing occurred. Baseline values were then measured and all participants in the Cr and PL groups engaged in 4 weeks of HIIT training on a cycle ergometer. Training consisted of either five or six sets of 2-minutes work with 1-minute passive rest, five days per week. Training intensity followed an undulating model starting at 80% PPO and reaching 120%. Significant two-way interactions [time (base- vs. post-) × treatment (Cr vs. PL vs. CON)] were discovered for CP (p = 0.007). Follow-up analyses indicated that the Cr group increased 6.72% ± 2.54%, while PL and CON showed no significant change in CP (3.87% ± 2.30% and -6.27% ± 2.38%, respectively). Furthermore, no changes in AWC were observed in any of the groups following treatment. The current findings suggest that the HIIT and supplementation protocol may be an effective method for improving CP, but had no effect on AWC. It would appear that Cr supplementation may enhance the effect of intense interval endurance training on aerobic performance changes as measure by the CP test. ACKNOWLEGMENTS: This investigation was supported by FSI Nutrition, Omaha, Nebraska.