Single muscle fibre metabolites and pulmonary oxygen uptake (VO2) were measured during moderate and intense, sub-maximal exercise to test the hypothesis that additional fibre recruitment is associated with the slow component of VO2. Seven healthy, male subjects performed 20min moderate (MOD, ~50% of VO2,max) and intense (INT, ~80% VO2,max) cycling at 70rpm. Glycogen content decreased significantly in type I and IIa fibres during INT, but only in type I fibres during MOD. During INT, creatine phosphate (CP) content decreased significantly both in types I and II fibres in the first 3min (��CP: 16.02.7 and 16.84.7mmol kg1 d.w., respectively) and in the next 3min (��CP: 16.24.9 and 25.76.7mmol kg1 d.w., respectively) with no further change from 620min. CP content was below the pre-exercise level (mean1SD) in 11, 37, 70 and 74% of the type I fibres after 0, 3, 6 and 20min of INT, respectively, and in 13, 45, 83 and 74% of the type II fibres. During INT, VO2 increased significantly by 61 and 41% in the periods 36 and 620min, respectively (��VO2,(63min): 0.140.02lmin1), whereas VO2 was unchanged from 3 to 20min of MOD. Exponential fitting revealed a slow component of VO2 during INT that appeared after ~2.6min and amounted to 0.24lmin1. The present study demonstrates that additional type I and II fibres are recruited with time during intense sub-maximal exercise in temporal association with a significant slow component of VO2.