The anaerobic energy release during submaximal arm (AE) and leg exercise (LE) has been estimated from O2 deficit measured at the onset of exercise. Eight male subjects were studied during 8-10 min of arm or leg cycling at the same relative workload (53% of the peak exercise-induced increase in pulmonary oxygen uptake, VO2). The workloads were 78 +/- 4 W during AE and 173 +/- 11 W during LE and VO2 was 1.51 +/- 0.06 1 min-1 for AE and 2.33 +/- 0.15 1 min-1 for LE. The half-time of the VO2 on-response was considerably longer (P < 0.01) during AE (62 +/- 9 s) than during LE (33 +/- 4 s) and the peak blood lactate concentration was higher (P < 0.05) during AE (4.8 +/- 0.5 mmol.l-1) than during LE (3.5 +/- 0.4 mmol.l-1). Oxygen deficit was 1.64 +/- 0.16 and 1.78 +/- 0.16 1 for AE and LE respectively. Oxygen deficit was higher during AE than during LE when related to absolute workload (P < 0.01), or to VO2 at steady state (P < 0.001) or to limb volume (P < 0.001). The proportion of the total energy demand covered by anaerobic energy release at the onset of exercise (0-8 min) was about 54% higher (P < 0.01) during AE than during LE. It is concluded that the energy release to a greater extend is covered by anaerobic processes during AE than during LE.