Previous research investigating endurance sports from a physiological perspectivehas mainly used constant or graded exercise protocols, although the nature ofsports like cross-country skiing and road cycling leads to continuous variations inworkload. Current knowledge is thus limited as regards physiological responses tovariations in exercise intensity. Therefore, the overall objective of the present thesiswas to investigate cardiovascular and metabolic responses to fluctuations inexercise intensity during exercise. The thesis is based on four studies (Studies I-IV);the first two studies use a variable intensity protocol with cardiorespiratory andblood measurements during cycling (Study I) and diagonal skiing (Study II). InStudy III one-legged exercise was used to investigate muscle blood flow duringvariable intensity exercise using PET scanning, and Study IV was performed toinvestigate the transition from high to low exercise intensity in diagonal skiing,with both physiological and biomechanical measurements. The current thesisdemonstrates that the reduction in blood lactate concentration after high-intensityworkloads is an important performance characteristic of prolonged variableintensity exercise while cycling and diagonal skiing (Studies I-II). Furthermore,during diagonal skiing, superior blood lactate recovery was associated with a highaerobic power (VO2max) (Study II). Respiratory variables such as VE/VO2, VE/VCO2and RER recovered independently of VO2max and did not reflect the blood lactate oracid base levels during variable intensity exercise during either cycling or diagonalskiing (Studies I-II). There was an upward drift in HR over time, but not inpulmonary VO2, with variable intensity exercise during both prolonged cyclingand diagonal skiing. As a result, the linear HR-VO2 relationship that wasestablished with a graded protocol was not present during variable intensityexercise (Studies I-II). In Study III, blood flow heterogeneity during one-leggedexercise increased when the exercise intensity decreased, but remained unchangedbetween the high intensity workloads. Furthermore, there was an excessiveincrease in muscular VO2 in the consecutive high-intensity workloads, mainlyexplained by increased O2 extraction, as O2 delivery and blood flow remainedunchanged. In diagonal skiing (Study IV) the arms had a lower O2 extraction thanthe legs, which could partly be explained by their longer contact phase along withmuch higher muscle activation. Furthermore, in Study IV, the O2 extraction in botharms and legs was at the upper limit during the high intensity workload with nofurther margin for increase. This could explain why no excessive increase inpulmonary VO2 occurred during diagonal skiing (Study II), as increased O2extraction is suggested to be the main reason for this excessive increase in VO2(Study III).