The kinetics of oxygen uptake (VO2) were observed at the onset of submaximal cycling exercise in seven men and one woman [mean age 22.6 +/- 0.9 (SE) yr] in the upright and supine positions and the supine position with -40 mmHg lower body negative pressure (LBNP). There was no significant difference for peak VO2 and ventilatory threshold between the supine (3,081 +/- 133 and 1,954 +/- 138 ml/min, respectively) and the supine + LBNP positions (3,062 +/- 152 and 1,973 +/- 122 ml/min); however, both were reduced compared with upright exercise (3,483 +/- 200 and 2,353 +/- 125 ml/min). Kinetic analysis applied to six repetitions by each subject indicated a slowing from a mean total lag time (time required to achieve 63% of the difference in VO2 between baseline and new steady state) of 36.3 +/- 2.7 s in upright exercise to 44.1 +/- 3.5 s in the supine position. However, total lag time for the supine + LBNP position (36.0 +/- 2.8 s) did not differ from upright exercise but was significantly faster than supine exercise. These data have been interpreted in support of an O2 transport limitation to VO2 kinetics at the onset of supine exercise that is countered by LBNP, likely through a more rapid increase in perfusion to the exercising muscle at these submaximal work rates.