PURPOSE: Time resolved near-infrared spectroscopy (TR-NIRS) can accurately and non-invasively measure cerebral O2 saturation (stO2). While several studies report this measurement during incremental exercise, little is known about responses to interval-type exercise, which more closely represent real-life exercise engagement. Further, the potential role of cerebral hemodynamics/oxygenation in regulating exercise performance in children is unknown, and stO2 patterns in the pre-frontal cortex (PFC) at high intensities may reflect early perception/onset of fatigue. Here, we compared the measured reductions in stO2 during incremental exercise with oscillations in stO2 during interval exercise in children. METHODS: 9 children (6 F) completed 2 cycle ergometry tests: 1. A maximal test to exhaustion, with pedaling resistance increased continuously over 10-12 min (ramp test); 2. A series of 10 2-minute bouts at ∼ 80% of peak VO2, punctuated by 1-minute rests (10x2). For all testing, a TR-NIRS probe (TRS-20, Hamamatsu) was placed on the forehead to record continuously PFC stO2. For ramp tests, the drop in stO2 was calculated between maximum value (2-5 min after start), and stO2 value at 80% peak VO2, as well as the difference between maximum and lowest stO2, typically recorded at peak VO2. In 10x2 tests, the magnitude of stO2 oscillations (max to min) for each bout was calculated, and values compared. RESULTS: On average, during ramp tests at 80% of peak stO2 dropped by 3.3 ± 1.2 % points below the maximum value, and at peak exertion by 7.4 ± 1.7 % points. During interval exercise, the lowest drop was measured during the 3rd bout (3.3 ± 1.0 % points), then gradually increased and stabilized during the last 4 bouts, when it ranged from 4.5±.8 to 4.8±1.0 % points. The average maximum individual drop was 5.6 ± .8 %. CONCLUSIONS: Both ramp and 10x2 exercise are associated with drops in PFC stO2, reflecting changes in cerebral oxygenation. During ramp exercise the magnitude of the stO2 drop appears proportional to work intensity. During interval exercise, the stO2 drop is initially similar to that observed at comparable work rates during the ramp test, and then gradually increases without however reaching the maximal drop recorded at peak exertion. These finding may have implications in determining the cognitive component of exercise participation/performance in different subject groups. Support: NIH Grants NICHD P01HD048721 & UL1 TR000153