The purpose of this study was to compare energy expenditure and substrate utilization during 60 min of steady state-exercise at similar heart rates (HR) using four exercise modes: stationary cycle (C), rower (R), ski simulator (S), and treadmill (walking) (T). Five subjects (means age = 23 +/- 4 yr) performed 60 min of continuous exercise at 65% HR max on each of the four modes in random order. Total energy (TE) and fat energy (FE) expenditure were determined from VO2 and respiratory exchange ratio (RER). VO2 during exercise averaged 2.427 for C, 2.167 for R, 2.242 for S, and 2.420 l min-1 for T and were not significantly different by repeated measures ANOVO (p greater than 0.05). RER, TE, and FE also were not statistically different among exercise modes. However, walking and skiing tended to use more fat; the average 60 min cumulative exercise values were 960 for C, 871 for R, 1088 for S, and 1188 kJ for T. The rate of fat expenditure generally increased after 20 min on all modes. These results indicate that the energy expended at comparable relative HR's is similar for four aerobic exercise modes.