It was the purpose of this study to examine the effect of a priming contractile bout on oxygen uptake (VO2) on-kinetics in highly oxidative skeletal muscle. Canine gastrocnemii (n=12) were stimulated via their sciatic nerves (8 V, 0.2-ms duration, 50 Hz, 200-ms train) at a rate of 2 contractions/3 s (approximately 70% peak VO2) for two 2-min bouts, separated by 2 min of recovery. Blood flow was recorded with an ultrasonic flowmeter, and muscle oxygenation monitored via near-infrared spectroscopy. Compared with the first bout (bout 2 vs. bout 1), the VO2 primary time constant (mean+/-SD, 9.4+/-2.3 vs. 12.0+/-3.9 s) and slow-component amplitude (5.9+/-6.3 vs. 12.1+/-9.0 ml O2.kg wet wt(-1).min(-1)) were significantly reduced (P<0.05) during the second bout. Blood flow on-kinetics were significantly speeded during the second bout (time constant=7.7+/-2.6 vs. 14.8+/-5.8 s), and O2 extraction was greater at the onset of contractions (0.050+/-0.030 vs. 0.020+/-0.010 ml O2/ml blood). Kinetics of muscle deoxygenation were significantly slower at the onset of the second bout (7.2+/-2.2 vs. 4.4+/-1.2 s), while relative oxyhemoglobin concentration was elevated throughout the second bout. These results suggest that better matching of O2 delivery to VO2 speeds Vo(2) on-kinetics at this metabolic rate, but do not eliminate a potential role for enhanced metabolic activation. Additionally, altered motor unit recruitment at the onset of a second bout is not a prerequisite for reductions in the VO2 slow-component amplitude after a priming contractile bout in canine muscle in situ.