In six subjects, the transformation of phosphorylase and glycogenolytic rate were studied in the first and third of three 30-s bouts of maximal cycling, separated by 4 min of rest. Vastus biopsies were taken before, at 15s and 30s of bouts 1 and 3. In bout 1, the mole fraction of active phosphorylase (PHOS a) was 9.9+2.2% at rest, increased to 47.4±6.4% at 15s and by 30s, decreased to 21.8+4.2%. During the first and last 15s of bout 1, glycogenolytic rates were 183 and 34 mmol/min/kg dw, respectively. The 15s [AMP,] of 3.8+0.6 umol/kg dw and [P,2-] of 29.1±5.7 mmol/kg dw were 20.0+0.3 and 28.7+3.5, respectively at 30s. [H] was 157.4+9.8 at 15s and 182.9±12.4 nmol/l at 30s. Bout 3 resting PHOS a of 10.5+2.7% increased to 20.5+1.6% at 15s and decreased to 16.2+5.7% at 30s. Glycogenolytic rates were 28 and 11 mmol/min/kg dw during the first and last 15s of bout 3, respectively. The bout 3 15s [AMP,] of 5.1 ±1.6 Mmol/kg dw and [P,2-] of 28.3+5.0 mmol/kg dw, were 15.6+4.1 and 26.5+5.9, respectively at 30s. [H4] was 231.4+13.0 and 254.5+36.1 nmol/l at 15s and 30s, respectively. Elevated [HJ during the last 1?s of bout 1 may result in decreased PHOS a and glycogenolysis despite increased [AMPJ and [P,2"]. Both PHOS a and glycogenolysis are further reduced in bout 3 compared to bout 1, despite similar [AMPJ and [P,2-] suggesting [H] is again responsible for the decreased flux. Funded by MRC and NSERC.