In healthy animals under normotensive conditions, perfusion of contracting skeletal muscle is regulated precisely to maintain microvascular O₂ pressures (PmvO₂) at levels commensurate with O₂ demands (Behnke et al. Respir. Physiol. 2001). Hypovolemic hypotension impairs muscle contractile function and we tested the hypothesis that this condition would alter the matching of O₂ delivery (QO₂) to O₂ utilization (VO₂) as determined by PmvO₂ at the onset of muscle contractions. PmvO₂ in the spinotrapezius muscles of seven female Sprague-Dawley rats (body mass, 280±6 g) was measured every 2 s across the transition from rest to 1 Hz twitch contractions using phosphorescence quenching techniques. Spinotrapezius blood flow (via radioactive microspheres) and O₂ uptake (VO₂m) were determined at rest and during the contracting steady-state in a second group of 7 rats. Measurements were made under normotension (N, mean arterial pressure, MAP, 97±4 mmHg) and hypotension (H, induced by arterial section, MAP, 58±3 mmHg, P<0.05) and the PmvO₂ profiles were modeled using a multi-compartment exponential fitting with independent time delays (KaleidaGraph 3.5). Hypotension reduced muscle blood flow at rest (N, 24±8 vs. H, 6±1 ml/min/100 g; P<0.05) and during contractions (N, 74±20 vs. H, 22±4 ml/min/100 g; P<0.05). H significantly decreased resting PmvO₂ and the steady-state contracting PmvO₂ (N, 19.4±2.4 vs. H, 8.7±1.6 mmHg, P<0.05). At the onset of contractions H reduced the time delay (N, 11.8±1.7 vs. H, 5.9±0.9 s, P<0.05) prior to the fall in PmvO₂ and accelerated the rate of PmvO₂ decrease (time constant, N, 12.6±1.4 vs. H, 7.3±0.9 s, P<0.05). The majority of H responses (6 rats) evidenced an early and pronounced fall of PmvO₂ to values below the steady-state that was not present in the N condition. VO₂m was reduced by 71% at rest and 64% with contractions in H versus N, and O₂ extraction during H averaged 78% at rest and 94% during contractions vs. 51 and 78% in N. These results demonstrate that hypovolemic hypotension constrains the increase of skeletal muscle QO₂ relative to that of VO₂ at the onset of contractions leading to a decreased PmvO₂. According to Ficks law, this scenario will decrease blood-myocyte O₂ flux thereby slowing VO₂ kinetics and exacerbating the O₂ deficit generated at exercise onset.