This thesis aimed to investigate and model the vo2, response to exhaustive constant intensity running of a short duration. Study I was a Douglas bag based study that compared the vo2, response of physically active subjects to 2, 5 and 8 minute bouts of constant intensity exhaustive exercise performed in both cycling and running. Nine male subjects took part and each completed a ramp test in addition to the three constant intensity tests in both modes of exercise. In the 5 and 8minute tests the subjects achieved 97.0 ± 4.2 and 97.5 ± 2.0 % for cycling, and 98.5 ± 1.8 and 99.2 ± 2.3% for running, of the ramp test Vo,peak. In the 2 minute test, a significantly lower percentage was attained (89.9 ± 5.5% and 91.8 ± 2.5% for cycling and running respectively). In cycling VO, was still increasing over the final minute of the test, whereas in running there was no difference between the last two 30 second samples (P~0.98). It was concluded that in severe intensity exercise of a short duration VO2 may not achieve its maximum and· that in running it may plateau at this sub-maximal rate. Study 2 validated the QP9000 for the measurement of vo2, during running on a breath-by-breath basis. Six male subjects performed a ramp test and tests at rest and at moderate and severe intensities. Each test was performed twice, once using the QP9000 and once using a Douglas bag system. No difference was found for VO, between the two systems (P~0.358). The SD of the differences between the systems across exercise intensities was 97 ± 57 ml.min·1 It was concluded that the QP9000 provides a valid measure of VO, at all exercises intensities. Study 3 investigated the vo2, response of trained runners to 800 m pace running, following a track based time trial. Eight male subjects (Vo,max 68.8 ± 5.6 m1.kg·'min-') took part in the study. That vo2, reached a plateau below Vo, was confirmed by a gradient of -29 ± 275 ml.min-' during the final 30 seconds of exercise. The asymptotic vo2, was only 85.3 ± 6.6 % of the Vo,peak from a ramp test and the response was shown to be extremely fast (time constant (T) of 10.7 ± 3.4 seconds). These breath-by-breath data confirmed the response indicated in Study I. Study 4 explored the single exponential model used to describe the vo2, response and the nature and level of breath-by-breath noise in severe intensity running. Five male subjects performed a ramp test to determine the speed at anaerobic threshold (AT). Each subject then performed five 8-minute runs at a speed corresponding to 90% of the AT and five exhaustive runs at a speed that would elicit exhaustion in about 2-minutes. Analysis of the noise to signal ratio of the severe intensity data showed that the noise was Gaussian and that averaging data over repeated transitions reduced this ratio. Computer simulations of noise equivalent to the noisiest subject's data demonstrated that the use of two repeated severe intensity transitions would give 95% confidence limits of < ± 1.2 seconds for T. Study 5 examined the effect of prior supra AT exercise on the VO, response to exhaustive severe intensity running. Ten middle and long distance runners each completed a ramp test to determine AT and Vo,peak. Subjects then ran exhaustive transitions, lasting approximately 2-minutes, that were preceded by moderate (90% AT) or heavy (50% of the difference between AT and Vo,peak) intensity running. Each transition was repeated. Increased metabolic acidosis (from prior heavy intensity exercise) did not increase the asymptotic vo2, (P ~ 0.226), and this figure represented only about 90% of Vo,peak from the ramp test. The mean response time (MRT) (time to reach 63% of the overall response) was faster following heavy exercise (20.9 ± 1.9 s vs. 18.9 ± 1.0 s, P<0.05). This was however due to a reduction in the duration of the initial (cardiodynamic) phase of the response rather than a speeding of the primary (phase 2) kinetics. Study 6 analysed the differences in this response between sprint and endurance runners. Six male athletes were recruited for each group based on best times for 100 m and 10 000 m. Subjects performed repeated transitions at a speed that would elicit exhaustion in approximately 2-minutes. No difference was found in the model of the VO, response between groups. When all subjects were analysed however a strong negative relationship was demonstrated between Vo,peak (from a ramp test) and the percentage of this Vo,peak that was reached in the constant speed test (r ~ -0.811, P ~ 0.001). It was concluded that the VO, response was dependent on the aerobic capabilities of the individual. In conclusion the thesis demonstrated a vo2, response in trained subjects during exhaustive severe intensity running that was different to that which is suggested by the majority of the literature. The vo2, response tends neither to VO, required nor to Vo,max, but rather tends to a plateau that is sub-maximal. This thesis was unable to identity the mechanisms that might result in such a shortfall in the response. However there appears to be a close link with the aerobic capabilities of the individual.