The mechanism(s) responsible for the increase in cerebral blood flow velocity (CBFV) at the onset of exercise is unknown but may relate to the arterial lactate-to-pyruvate ratio(L/P). In order to evaluate this hypothesis, blood was drawn from the radial artery and the internal jugular vein immediately before and during the first 120 s of ergometer cycling in 8 young trained males. The CBFV was appreciated with transcranial ultrasound Doppler. At rest, mean heart rate (HR) was 78 ± 15 (SD) bpm, mean arterial pressure (MAP) 108 ± 16 mmHg and CBFV 42 ± 9 cm s-1. The arterial lactate concentration was 1.1 ± 0.3 mM, that of pyruvate 85 ± 45 µM and the L/P ratio 13.1 ± 4.2, and similar to the jugular vein (13.5 ± 2.3). Five seconds after the onset of exercise, HR was 100 ± 19 bpm, MAP 104 ± 17 mmHg, CBFV 43 ± 11 cm s-1 and the arterial and venous L/P ratio 13.1 ± 3.8 and 12.9 ± 1.2, respectively. The CBFV stabilised ~25% above baseline after 50 s of exercise, while the arterial and venous L/P ratio increased to 15.3 ± 2.7 and 14.7 ± 1.6, respectively. Approximately 25% of the change in CBFV after 5 s and ~50% after 60 s of exercise could be attributed to changes in plasma L/P ratio. These results qualify the arterial L/P ratio as one possible signal for exercise induced elevation of cerebral blood flow.