The response of respiratory gas exchanges to a 6 week high intensity training program was examined in 5 healthy males during fixed term maximal incremental treadmill exercise. Training was performed 3 d·wk^–1 and consisted of a progressive series of repeated 15 sec and 30 sec maximal runs, and weight training exercises for the leg extensor muscles. Respiratory gases during the tests were continuously monitored using an on-line system. Muscle biopsy samples were obtained from the m. vastus lateralis before and after training for histochemical determination of fibre distribution based on myosin ATP-ase activity, and fibre cross-sectional area based on NADH-Tetrazolium Reductase activity. Training significantly increased the proportion of type IIa fibres (+5.9±2.0%,p<0.001) and decreased type I fibres (–6.3±2.0%,p<0.001), the distribution of type IIb fibres remained unchanged (+0.4±0.9%). Muscle cross-sectional area also showed a significant increase after training in type I (+ 318±215 [(V)\dot]_{\textCO\text2} \dot V_{{\text{CO}}_{\text{2}} } ) and mean respiratory exchange ratio ( R = [(V)\dot]_{\textCO\text2} /[(V)\dot]_\textO\text2 R = \dot V_{{\text{CO}}_{\text{2}} } /\dot V_{{\text{O}}_{\text{2}} } ) were significantly increased (p<0.01) after training. The R-time relationship was at all times shifted to the left after training, being significantly (p<0.01) so over the final five min of exercise. No changes in mean exercise oxygen uptake ( [(V)\dot]_\textO\text2 \dot V_{{\text{O}}_{\text{2}} } ), maximum oxygen uptake ( [(V)\dot]_{\textO\text2 \textmax} \dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } ) and maximum heart rate (FHR_max) were observed between tests. These results indicate that high intensity training can significantly affect respiratory exchange during fixed term progressive exercise.