The rate of metabolism in forearm flexor muscles (MO2) was derived from near-infrared spectroscopy (NIRS-O2) during ischaemia at rest rhythmic handgrip at 15% and 30% of maximal voluntary contraction (MVC), post-exercise muscle ischaemia (PEMI), and recovery in seven subjects. The MO2 was compared with forearm oxygen uptake (VO2) [flow x (oxygen saturation in arnterial blood-oxygen saturation in venous blood, SaO2 - SvO2)], and with the 31P-magnetic resonance spectroscopy-determined ratio of inorganic phosphate to phosphocreatine (P(I):PCr). During ischaemia at rest, the fall in NIRS-O2 was more pronounced [76 (SEM 3) to 3 (SEM 1)%] than in SvO2 [71 (SEM 3) to 59 (SEM 2)%]. During the handgrip, NIRS-O2 was lower at 30% compared to 15% MVC [58 (SEM 3) v.s. 67 (SEM 3)%] while the SvO2 was similar [29 (SEM 3) v.s. 31 (SEM 4)%]. Accordingly, MO2 as well as P(I):PCr increased twofold, while VO2 increased only 30%. During PEMI after 15% and 30% MVC, NIRS-O2 fell to 9 (SEM 1)% and "0", but the use of oxygen by forearm muscles was not reflected in SvO2. During reperfusion after PEMI, the peak NIRS-O2 was lowest after intense exercise, while for SvO2 the reverse was seen. The discrepancies between NIRS-O2 and SvO2, and therefore between the estimates of the metabolic rate, would suggest significant limitations in sampling venous blood which is representative of the flexor muscle capillaries. In support of this contention, SvO2 and venous pH decreased during the first seconds of reperfusion after PEMI. To conclude, NIRS-O2 of forearm flexor muscles closely reflected the exercise intensity and the metabolic rate determined by magnetic resonance spectroscopy but not that rate derived from flow and the arterio-venous oxygen difference.