The purpose of this study was to determine the effects of mode-specific warm-up of three different durations (0, 5 and 10 min) on gross mechanical efficiency during a subsequent bout of steady-state exercise. Fifteen untrained males (M age=20.4 + 1.0 yrs, M VO2peak=44.4 + 10.4 ml/kg/min) served as subjects and reported to the laboratory on four separate occasions, separated by 48-72 h. The first laboratory session included an orientation as well as a test of peak aerobic power using a cycle ergometer with a standard protocol. Oxygen uptake (VO2) and respiratory exchange ratio (RER) were monitored continuously using open-circuit spirometry. Heart rate (HR) was monitored using a Polar monitor. The test continued to exhaustion. Power outputs corresponding to 40 and 70% VO2 peak were then determined using individual plots of VO2 vs. power output. The second, third and fourth sessions consisted of six min of cycling at power outputs corresponding to 70% VO2peak preceded by one of the following, presented in counterbalanced sequence: 5 min at 40%VO2peak, 10 min at 40% VO2peak or 10 min seated rest. VO2 and RER were recorded every 30 s and averaged for each min. HR was also recorded each min. Power outputs for each one-min interval were converted to kcal/min and totaled for each six-min bout. This served as the numerator for the gross mechanical efficiency calculations. VO2 for each one- min interval was multiplied by the caloric equivalent of O2 for each corresponding RER to arrive at metabolic energy cost (kcal/min) which was then totaled for each six-min bout. This value served as the denominator. Gross mechanical efficiencies were analyzed using a one-way ANOVA. Significant differences (p<.05) in gross mechanical efficiency were found between the 0 (20.06 + 1.54%) and 5 (22.49 + 3.07%) but not between the 0 and 10 (21.38 + 2.76%) or 5 and 10. No significant differences (p>.05) in HR were found among the trials. It was concluded that 5 min of mode-specific warm-up performed at an intensity of 40% VO2 peak produces significant improvements in gross mechanical efficiency during subsequent cycle ergometer exercise at an intensity of 70% VO2 peak. Keyword(s): adult fitness, exercise/fitness