Reductions in systemic and locomotive skeletal muscle blood flow and O₂ delivery limit maximal aerobic capacity during constant maximal exercise in humans (González-Alonso & Calbet 2003; González-Alonso et al. 2004). This study examined whether systemic O₂ delivery limits maximal aerobic power (VO_2max) during incremental exercise to exhaustion. We measured systemic haemodynamics, O₂ transport and O₂ uptake during incremental (20, 40, 60, 80, 90, 95, 100% of peak power output) and constant (372±11 W ∽ 85% of peak power output; 6.87±0.50 min; mean±SEM) cycle ergometer exercise to exhaustion in 8 trained male subjects (29±1 years, 4.85±0.1 l min^-1). Data were analysed by repeated measures one-way ANOVA and Tukey’s honestly significant difference (HSD) post hoc procedure. Statistical significance was accepted at P < 0.05. During incremental exercise, cardiac output and systemic O₂ delivery increased linearly up to 80% of peak power (both r²=0.998, P <0.001), peaked at ~92% of peak power and thereafter plateaued or decreased in parallel to a decline in stroke volume (P <0.05) and increases in central venous and mean arterial pressures. In contrast, heart rate and pulmonary VO₂ increased linearly until exhaustion (r²≥0.993; P <0.001) accompanying a continuous rise in systemic O₂ extraction to 84±2%. During constant load exercise, cardiac output, stroke volume and systemic O₂ delivery and uptake increased during the first ~5 min and dropped before exhaustion (all P <0.05) despite increasing or constant central venous and mean arterial pressures. The fall in systemic O₂ delivery in both maximal tests was solely owing to the decline in cardiac output because arterial O₂ content increased until exhaustion. In conclusion, these results in healthy trained humans indicate that maximal aerobic power is largely limited by the inability of the heart to sustain a linear increase in cardiac output and O₂ delivery. Furthermore, the similar impairment in stroke volume and systemic O₂ delivery during incremental and constant maximal exercise strongly supports a preponderant cardiac limitation to maximal aerobic power and capacity in humans. This work was supported by the Gatorade Sports Science Institute. All procedures accord with current local guidelines and the Declaration of Helsinki.