Oxygen uptake kinetics (VO2) are slowed when exercise is initiated from a raised metabolic rate. Whether this is due to recruitment of muscle fibres differing in oxidative capacity, or slowed blood flow (Q) kinetics is unclear. This study determined τVO2 in canine muscle in situ, with experimental control over muscle activation and Q during contractions initiated from rest and a raised metabolic rate. The gastrocnemius complex of 9 anaesthetised, ventilated dogs was isolated and attached to a force transducer. Isometric tetanic contractions (50Hz; 200ms duration) via supramaximal sciatic nerve stimulation were used to manipulate metabolic rate: 3 min stimulation at 0.33Hz (S1) followed by 3 min at 0.67Hz (S2). Circulation was initially intact (SPON), and subsequently isolated for pump-perfusion (PUMP) above the highest value in SPON. Muscle VO2 was determined contraction-by-contraction using an ultrasonic flowmeter and venous oximeter, and normalised to tension-time integral (TTI). τVO2/TTI and τQ were less in S1SPON (13±3s and 12±4s; mean±SD) than S2SPON (29±19s and 31±13s; P<0.05). τ VO2/TTI was unchanged by pump-perfusion (S1PUMP, 12±4s; S2PUMP, 24±6s; P<0.001) despite increased O2 delivery; at S2 onset, venous O2 saturation was 21±4% and 65±5% in SPON and PUMP respectively. VO2 kinetics remained slowed when contractions were initiated from a raised metabolic rate despite uniform muscle stimulation and increased O2 delivery. The intracellular mechanism may relate to a falling energy state, approaching saturating ADP concentration, and/or slowed mitochondrial activation; but further study is required. These data add to the evidence that muscle VO2 control is more complex than previously suggested.