Relationships between percentage of maximal oxygen consumption (%VO2max) and percentage of maximal heart rate reserve (%f(cr)) were compared during steady states of exercise (S), transitory states of exercise (T) and a 5-min recovery period (R). Male adults [mean age 27 (SD 10) years] were studied exercising on a treadmill (TR, n = 26), cycle ergometer (CE, n = 14) and arm traction bench (ATB, n = 14). The exercise intensity was adjusted according to the subjects in order to reach exhaustion in 4-5 steps of 2 min (ATB) or 3 min (TR, CE). The 1st min of each stage was considered as T and the last minute of each stage as S. The oxygen consumption (VO2) and heart rate (f(c)) were recorded simultaneously. Significant correlations were observed for each type of exercise and for each state between %f(cr) and %VO2max (r range 0.87-1.00). During T and R, the %VO2max versus %f(cr) relationships were laterally shifted, suggesting a resetting of f(c) control mechanisms. In S, the intercept was greater than in T and R; in T, the slope was greater than in S and R. The VO2 could be predicted from individual %VO2max versus %f(cr) relationships during T and R as is usually done in S using specific equations. Taking into consideration the average relationships established on the three ergometers, the standard error of the predicted VO2 during S and T reached 10%-20% and 22%-38% in R. During exercise, the higher the intensity the better was the prediction of VO2 from f(c) (r range 0.46-0.60, P < 0.001). Therefore except at high exercise intensities, it was found that individual relationships had to be used to obtain an accurate estimation of VO2.