Breath-by-breath (BbB) oxygen uptake rate (V(O)(2)) was measured at the mouth (MO) and at the alveolar level, at the onset of square wave cycling exercise of moderate intensity in six healthy male subjects. Alveolar BbB V(O)(2) values were calculated correcting MO V(O)(2) values by (i) estimating (GR); and (ii) measuring (opto-electronic plethysmography, OEP) BbB lung O(2) store changes.V(O)(2) kinetics was then described by a bi-exponential model. GR yielded larger values of the time constants (tau2) of the primary phase of V(O)(2) kinetics. The mean response times (MRTs) calculated by analysing GR BbB V(O)(2) values were larger than (i) those obtained by using MO and OEP at 90W; and (ii) that by using MO at 120W. OEP corrected V(O)(2) yielded the highest normalised amplitude of the cardiodynamic phase of the V(O)(2) on-response. Correction of BbB V(O)(2) for actual BbB changes of lung O(2) stores by OEP thus seems more appropriate for the study of the early cardiodynamic phase of V(O)(2) kinetics than GR.