Seventeen young healthy physically active males (age 23 ±3 years; body mass (BM) 72.5 ±7.9 kg; height 178 ±4 cm, (mean ±SD)), not specifically trained in cycling, participated in this study. The subjects performed two cycling incremental tests at the pedalling rate of 60 rev x min-1. The first test, with the power output (PO) increases of 30 W every 3 min, was to determine the maximal oxygen uptake (V'O2max) and the power output (PO) at V'O2max, while the second test (series of 6 minutes bouts of increasing intensity) was to determine energy expenditure (EE (V'O2)), gross efficiency (GE (V'O2/PO)) and delta efficiency (DE(ΔV'O2/DPO)) during sub-lactate threshold (LT) PO. V'O2max was 3.79 ±0.40 L x min-1 and the PO at V'O2max was 288 ±27 W. In order to calculate GE and DE the V'O2 was expressed in W, by standard calculations. GE measured at 30 W, 60 W, 90 W and 120 W was 11.6 ±1.4%, 17.0 ±1.4%, 19.6 ±1.2% and 21.4 ±1.1%, respectively. DE was 29.8 ±1.9%. The subjects' BM (range 59-87 kg) was positively correlated with V'O2 at rest (p<0.01) and with the intercept of the linear V'O2 vs. PO relationship (p<0.01), whereas no correlation was found between BM and the slope of V'O2 vs. PO. No correlation was found between BM and DE, whereas GE was negatively correlated with BM (p<0.01). GE was also negatively correlated with V'O2max and the PO at V'O2max (p<0.01). We conclude that: V'O2 at rest affects GE during moderate-intensity cycling and GE negatively corelates with V'O2max and the PO at V'O2max in young healthy men.