The hypothesis that the rapid increases in blood flow at the exercise onset are exclusively due to the mechanical effects of the muscle pump was tested in six volunteers during dynamic handgrip exercise. While supine, each subject completed a series of eight different exercise tests in which brachial artery blood pressure (BP) was altered by 25-30 mmHg (1 mmHg = 133.3 Pa) by positioning the arm above or below the heart. Two different weights, corresponding to 4.9 and 9.7% of maximal voluntary isometric contraction, were raised and lowered at two different contraction rate schedules (1s:1s and 2s:2s work-rest) each with a 50% duty cycle. Beat-by-beat measures of mean blood velocity (MBV) (pulsed Doppler) were obtained at rest and for 5 min following step increases in work rate with emphasis on the first 24 s. MBV was increased 50-100% above rest following the first contraction in both arm positions (p < 0.05). The increase in MBV from rest was greater in the below position compared with above, and this effect was observed following the first and subsequent contractions (p < 0.05). However, the positional effect on the increase in MBV could not be explained entirely by the approximately 40% greater BP in this position. Also, the greater workload resulted in greater increases in MBV as early as the first contraction, compared with the light workload (p < 0.05) despite similar reductions in forearm volume following single contractions. MBV was greater with faster contraction rate tests by 8 s of exercise. It was concluded that microvascular vasodilation must act in concert with a reduction in venous pressure to increase forearm blood flow within the initial 2-4 s of exercise.