Blood flow has long been hypothesized to be a primary determinant of arterial diameter growth. To verify this hypothesis, we placed a flow-restricting silver clip on one carotid artery and a completely nonocclusive clip on the contralateral artery in 13 rats (45-65 g). After 6-12 wk, measurements of vessel diameters and pulsed Doppler velocity wave-forms were obtained proximal to the clips, where flows differed from one side to the other, but mean pressures could be assumed approximately equal. Rats were anesthetized prior to these determinations and were fixed by perfusion at normal arterial pressure immediately thereafter. Flow reduction on the tightly clipped side averaged 35%. Flow-restricted arteries consistently showed reduction of diameter growth, averaging -10.2% of control diameters (P = 0.001). Thicknesses of the tunica media were similar on the flow-restricted and control sides. Side-to-side lumen circumference ratios were not correlated with mean blood flow velocities but were correlated with peak velocity (r = 0.62) and with an index of velocity pulsatility (r = 0.80). These results confirm a relationship between blood flow and arterial diameter and raise the possibility of a role for flow pulsatility.