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New Findings: What is the central question of the study?What is the reliability of middle cerebral artery velocity cerebrovascular reactivity (CVR) when using traditional and novel outcomes, as measured by transcranial Doppler?What is the main finding and its importance?Traditional CVR approaches presented acceptable reproducibility but should be expressed as an absolute CVR. Large within‐ and between‐individual differences in the middle cerebral artery velocity response profile support using a dynamic peak, rather than a set time point, for the most reliable interpretation. The study highlights the utility of novel kinetic CVR outcomes, but due to increased variability in time‐based metrics, this analysis requires larger sample sizes than traditional methods. Cerebrovascular reactivity (CVR) of middle cerebral artery velocity (MCAv) to CO2 is a common method to assess cerebrovascular function. Yet, the approaches used to calculate CVR outcomes vary. The aim of this study was to explore the within‐ and between‐day reliability of traditional CVR outcomes. The second aim was to explore the reliability of novel kinetic‐based analyses. Healthy adults (n = 10, 22.3 ± 3.4 years) completed assessments of CVR over 4 min using a fixed fraction of inspired CO2 (6%). This was repeated across four separate visits (between‐day), and on one visit measures were repeated 2.5 h later (within‐day). No mean biases were present between assessments for traditional CVR metrics, expressed as absolute (cm/s/mmHg) or relative (%/mmHg) outcomes (minute 3, minute 4, peak 1 s, peak 30 s) (between‐day: P > 0.14, ηp2 < 0.20; within‐day: P > 0.22, d > 0.27). Absolute, rather than relative, CVR yielded the most reproducible parameters (coefficient of variation: 8.1–13.2% vs. 14–83%, respectively). There were significant differences between CVR outcomes (P < 0.001, ηp2 > 0.89) dependent on the time point used to determine CVR, as a steady state MCAv response was rarely observed. Furthermore, the MCAv response was not reproducible within an individual (κ = 0.15, P = 0.09). No mean differences were present for novel kinetic outcomes (amplitude, time‐delay, time constant) (between‐day: P > 0.05, d < 0.33; within‐day: P > 0.38, d < 0.25). The results support the need for standardisation and indicate CVR should be defined as a dynamic peak, rather than a set time point for increased reliability. For novel kinetic outcomes variability was greater (CV: 8.7–120.9%) due to the nature of time‐based metrics. [ABSTRACT FROM AUTHOR]