Because the cardiocirculatory response of heart transplant recipients (HTR) to exercise is delayed, we hypothesized that their O2 uptake (V˙o 2) kinetics at the onset of subthreshold exercise are slowed because of an impaired early “cardiodynamic” phase 1, rather than an abnormal subsequent “metabolic” phase 2. Thus we compared the V˙o 2 kinetics in 10 HTR submitted to six identical 10-min square-wave exercises set at 75% (36 ± 5 W) of the load at their ventilatory threshold (VT) to those of 10 controls (C) similarly exercising at the same absolute (40 W; C40W group) and relative load (67 ± 14 W; C67W group). Time-averaged heart rate, breath-by-breathV˙o 2, and O2pulse (O2p) data yielded monoexponential time constants of the V˙o 2 (s) and O2p increase. Separating phase 1 and 2 data permitted assessment of the phase 1 duration and phase 2 V˙o 2 time constant ([Formula: see text]). The V˙o 2 time constant was higher in HTR (38.4 ± 7.5) than in C40W (22.9 ± 9.6; P ≤ 0.002) or C67W (30.8 ± 8.2; P ≤ 0.05), as was the O2p time constant, resulting from a lower phase 1V˙o 2 increase (287 ± 59 vs. 349 ± 66 ml/min; P ≤ 0.05), O2p increase (2.8 ± 0.6 vs. 3.6 ± 1.0 ml/beat; P ≤ 0.0001), and a longer phase 1 duration (36.7 ± 12.3 vs. 26.8 ± 6.0 s; P≤ 0.05), whereas the[Formula: see text]was similar in HTR and C (31.4 ± 9.6 vs. 29.9 ± 5.6 s; P = 0.85). Thus the HTR have slower subthresholdV˙o 2 kinetics due to an abnormal phase 1, suggesting that the heart is unable to increase its output abruptly when exercise begins. We expected a faster[Formula: see text]in HTR because of their prolonged phase 1 duration. Because this was not the case, their muscular metabolism may also be impaired at the onset of subthreshold exercise.