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It has been suggested that time at a high fraction (%) of maximal oxygen uptake (VO 2max ) plays a decisive role for adaptations to interval training. Yet, no study has, to date, measured the % of VO 2max during all interval sessions throughout a prolonged training intervention and subsequently related it to the magnitude of training adaptations. Thus, the present study aimed to investigate the relationship between % of VO 2max achieved during an interval training intervention and changes in endurance performance and its physiological determinants in well-trained cyclists. Twenty-two cyclists (VO 2max 67.1 (6.4) mL·min -1  ·kg -1 ; males, n = 19; females, n = 3) underwent a 9-week interval training intervention, consisting 21 sessions of 5 × 8-min intervals conducted at their 40-min highest sustainable mean power output (PO). Oxygen uptake was measured during all interval sessions, and the relationship between % of VO 2max during work intervals and training adaptations were investigated using linear regression. A performance index was calculated from several performance measures. With higher % of VO 2max during work intervals, greater improvements were observed for maximal PO during the VO 2max test (R 2  = 0.44, p = 0.009), PO at 4 mmol·L adjusted  = 0.44, p = 0.009), PO at 4 mmol·L -1 [blood lactate] (R 2 adjusted  = 0.25, p = 0.035), the performance index (R 2 adjusted  = 0.36, p = 0.013), and VO 2max (R 2 adjusted  = 0.54, p = 0.029). Other measures, such as % of maximal heart rate, were related to fewer outcome variables and exhibited poorer session-to-session repeatability compared to % of VO 2max . In conclusion, improvements in endurance measures were positively related to the % of VO 2max achieved during interval training. Percentage of VO 2max was the measure that best reflected the magnitude of training adaptations., (© 2024 The Author(s). European Journal of Sport Science published by Wiley‐VCH GmbH on behalf of European College of Sport Science.)