No previous scientific investigation has studied the influence of either increasing or decreasing levels of body weight (BW) support on oxygen consumption (VO2) while running in reduced BW conditions via a differential air pressure (DAP) treadmill. Moran et al. found rate of perceived exertion to be higher at the same BW conditions when moving from high BW support to low BW support but did not quantify the findings with VO2. PURPOSE: The purpose of this study was to both define the physiological changes, if any and perceived efforts, if any, between unloading and loading subjects on DAP treadmill to define loading or unloading order. Specifically, the findings of the study would contribute to our understanding of the potential factors that influence perceived effort. METHODS: Fifteen collegiate cross country runners (7M, 8F; 20.4 ± 2.4 yrs; 61.1 ± 12.6 kg) granted informed consent and completed all study procedures. Day one testing was a VO2 peak test (61.51 ± 14.8 ml/kg/min) run on an Alter-G treadmill (AGTM) (Fremont, CA) at 100% BW. Day two and three testing, counter-balanced, consisted of a 25-min run in which BW was either systematically increased (INC) or decreased (DEC) while velocity was held constant at 70% V02 peak. The DEC condition started at 100% BW and decreased 10% to 60% BW. The INC condition started at 60% BW and increased 10% to 100% BW. Each stage lasted 5-min for a total of 25- min. VO2 (Parvo True One 2400 Metabolic Cart) was collected over the last two-minutes of each stage, as was heart rate (HR; Polar RS 300). Rate of perceived exertion (RPE; Borg Scale) was recorded at the end of each stage. A paired t-test with Bonferroni adjustment was used to analyze the data. Statistics were processed with PASW software Ver. 16.0 (IBM Armonk, NY). RESULTS: There were no significant differences between conditions for VO2 and HR (p > 0.01). RPE data at 100% (11.2 INC- 8.4 DEC), 90%(10.4 INC- 8.6 DEC), and 80% (9.1 INC – 8.1 DEC) BW was significant (P< 0.002) CONCLUSION: From a physiological perspective there were no significant differences in VO2 or HR at any loading level, but participants perceived a greater demand at the 100% and 90% BW levels during the INC condition. Further research is needed to better understand the psychological differences when INC BW.