For constant-load, heavy exercise (i.e., above the lactate threshold (TLac.)), a slow component of oxygen uptake (&OV0312;O2) is observed. Endurance training reduces the magnitude of the slow component and, hence, end-exercise &OV0312;O2. Reductions in exercise &OV0312;O2have been reported after 7–8 wk of training; unpublished observations suggest that the &OV0312;O2slow component may be attenuated after just 2 wk of training. A minimum training intensity for eliciting reductions in constant-load exercise &OV0312;O2has not been established; however, in the elderly, training at an intensity below TLacresulted in similar reductions in exercise &OV0312;O2as did training above TLac. Mechanisms responsible for the reduced slow component of &OV0312;O2after training remain to be firmly established. Evidence both for and against blood lactate concentration ([L−]) as a mediator of the slow component has been published; high correlations between [L−] and the slow component, and between the training-induced reductions in these variables, appear to be more coincidental than causal. Decreased pulmonary ventilation after training may account for between 14% and 30% of the reduction in the slow component of &OV0312;O2. Epinephrine infusion does not augment exercise &OV0312;O2, nor does β-adrenergic blockade diminish the magnitude of the slow component of &OV0312;O2.