Adaptive responses of rabbit common carotid arteries were examined after 70-80% reductions in blood flow produced by ipsilateral external carotid artery ligation. These flow reductions elicited growth inhibition of arterial wall tissue in immature rabbits. Specifically, experimental carotid arteries exhibited DNA levels significantly lower, by 35%, than contralateral control arteries 1 mo after external carotid ligation. Lower elastin contents (38%) were also observed, although collagen contents were not affected. These changes were accompanied by a relative reduction in wall mass of 30% and a 31% reduction in internal diameter. Adult rabbits exhibited decreased internal diameter (21%) after flow reduction, but no significant change in vessel mass or wall constituents was observed. Early diameter reductions were vasoconstrictor in origin, but the vessel functioned as a smaller artery rather than as a partially constricted normal vessel after 1 mo, i.e., both maximally dilated and maximally constricted diameters were reduced. A reduction in endothelial cell number was detected for the narrowed vessels. Manipulation of local flow conditions indicated that the vessels responded to changes in mean blood flow rather than the pulsatile component of flow.