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Spontaneous and experimental changes in arterial blood flow rates affect tissue accumulation in developing arteries. To examine whether cell proliferation and/or cell death are affected by alterations in blood flow, we ligated the left external carotid artery of 3-week-old rabbits, which reduces left common carotid blood flow by 71%. In control arteries and after 2 days of flow reduction, agarose gel electrophoresis of DNA extracted from all carotid arteries resolved multiple low molecular weight bands characteristic of apoptosis; however, DNA fragmentation in arteries carrying reduced blood flow was 2.5-fold higher than that of control arteries. The effect of reduced blood flow on cell death subsequently waned but remained significant at 7 days. Cell death in carotid arteries was also detected by in vivo uptake of propidium iodide, a DNA-binding fluorescent dye that labels the nuclei of nonviable cells. Both smooth muscle and endothelial cells exhibited large and statistically significant increases in labeling index in the flow-reduced artery. Propidium iodide-labeled cells were cleared from the vessel wall within 1 to 4 hours of labeling, and nuclear staining displayed condensation (clumping) of chromatin in all labeled cells at later time points. This time course and nuclear morphology and the rapid clearance of labeled cells are consistent with death via apoptosis. Many propidium iodide-positive cells did not display chromatin condensation immediately after labeling; however, this was also true of cultured endothelial cells that were driven into apoptosis with sphingomyelinase treatment and then double-labeled with propidium iodide and the apoptosis marker annexin V. We infer that propidium iodide can label apoptotic vascular cells before these cells display chromatin condensation that is detectable with fluorescence labeling of DNA. Replication rates of smooth muscle and endothelial cells, determined by 5-bromo-2'-deoxyuridine uptake, were inhibited by >75% with decreased blood flow. The inhibition of proliferation was unabated after 7 days of reduced flow. These findings indicate that the coordinated regulation of cell death and cell proliferation, in response to changes in arterial blood flow rates, contributes to arterial remodeling during development.