Objectives: Affect experienced during high-intensity interval exercise (HIIE) is dependent on work-intensity, but the influence of increasing (low-to-high (L-H)) or decreasing (high-to-low (H-L)) work-intensity during HIIE remains unclear in adolescents. The role of prefrontal cortex haemodynamics in mediating changes in affect during HIIE also remains unexplored in adolescents. We examined affect, enjoyment and cerebral haemodynamic responses to HIIE with increasing or decreasing work intensities in adolescents. Methods: Participants (N=16; 8 boys; age 12.5±0.8 years) performed, on separate days, HIIE cycling consisting of 8 x 1-minute work-intervals at 100%-to-70% (HIIEH-L), 70%-to-100% (HIIEL-H) or 85% (HIIECON) peak power separated by 75 seconds recovery. Affect, enjoyment and cerebral haemodynamics (oxygenation (∆O2Hb), deoxygenation (∆HHb) and tissue oxygenation index (TOI)) were recorded before, during, and after all conditions. Results: Affect and enjoyment were lower during HIIEH-L compared to HIIEL-H and HIIECON at work-intervals 1 to 3 (all P<0.043, ES>0.83) but were greater during HIIEH-L than HIIEL-H and HIIECON at work-interval 8 (all P<0.048, ES>0.83). ∆O2Hb was similar across conditions (P=0.87) but TOI and ∆HHb were significantly greater and lower, respectively during HIIEH-L compared to HIIEL-H and HIIECON at work-interval 8 (all P<0.039, ES>0.40). Affect was correlated with TOI (all r>0.92) and ∆HHb (all r>-0.73) across conditions. Conclusions: HIIEH-L offers advancement to the HIIECON and HIIEL-H which bring significant greater affect and enjoyment toward the end HIIE work-interval, implicating the feasibility and adoption of this protocol for health promotion in youth. Also, changes in prefrontal cortex haemodynamics are associated with the affect during HIIE.