The between-day repeatability of simultaneous measures of brachial artery diameter (D) (echo Doppler) and mean blood velocity (MBV) (pulsed Doppler) was tested during rest and exercise. On 3 separate days, six volunteers performed one trial of 1-min rest followed by a step increase in dynamic handgrip exercise for 4 min which required the lifting and lowering of a 4.4-kg weight(≈8-12% MVC) in a 1s/2s (work/rest) cadence. Measures for MBV and D were collected continuously on a beat-by-beat basis during the transition from rest to end exercise. The mean rest values over one min, and single data points at 30, 60, 120, and 240 s of exercise were extracted from the time series data. At all exercise time points, MBV was greater than rest (P < 0.05), but these levels were not different across test days. Arterial D at all exercise time points ranged from 3.8 ± 0.1 mm to 4.1 ± 0.1 mm(mean ± SEM) and did not differ from rest (3.9 ± 0.1 mm)(P > 0.05), nor did D differ between days. The mean between-day coefficient of variation for D was 4.08 ± 0.7% at rest and ranged from 2.90 ± 0.4% to 3.96 ± 0.5% during exercise. The coefficient of variation for MBV was 13.2 ± 2.6% at rest and reached 20.2 ± 3.1% during the final min of exercise; the exercise variability was reduced to 14.9 ± 2.4% by averaging MBV over 3 s (the duration of a contraction/relaxation duty cycle) (P < 0.05) with no further advantage of averaging over ten 60-s sample periods. The data indicate that, for the six subjects tested, Doppler ultrasound measures of arterial MBV and diameter during both rest and exercise were reproducible across different test days and can be used as a reliable, noninvasive means of testing hypotheses pertaining to blood flow control.