OBJECTIVE The goal of this study was to determine whether release of endothelium-derived relaxing factor (EDRF) from carotid artery in response to acetylcholine is altered by aging. METHODS Responses were examined in arteries from Wistar rats 6 to 8 months old (young rats), 24 to 26 months old (old rats), and 30 to 32 months old (very old rats). We used a bioassay technique to measure release of EDRF from the carotid artery (donor vessel) and, in the same experiment, measured the diameter of pressurized donor arteries. RESULTS Responses of the donor arteries and the detector vessels (aortas) to sodium nitroprusside were not altered in old (24 to 26 months) and very old (30 to 32 months) rats compared with responses in the young rats. Dilator responses in the carotid artery to acetylcholine tended to be lower in the old rats and were significantly lower in very old rats compared with the young rats. Relaxation of the detector vessel during administration of acetycholine to the donor artery was not significantly different in young, old, and very old rats. CONCLUSIONS Endothelium-dependent dilatation in carotid arteries of old rats is impaired, but release of EDRF and responsiveness of vascular muscle to nitroprusside are normal. Thus, impairment of endothelium-dependent relaxation in the carotid artery during aging is not due to impaired release of EDRF.