1. The purpose of the present study was to determine the relationship between plasma and tissue lipid levels and the effects of age on vascular responses to noradrenaline (NA) and acetylcholine (ACh). 2. Studies were performed in young and aged rats and the response of endothelium-intact and -denuded aortic rings to NA and to ACh was measured. The plasma concentration of cholesterol (total, high-density lipoprotein (HDL) and low-density lipoprotein (LDL)) and 17 beta-oestradiol was determined, as was the aortic tissue content of phospholipids, cGMP and cholesterol (total, free and esterified). 3. Levels of all types of cholesterol in plasma and aorta increased with age; cholesterol levels in plasma correlated with those in the aorta; levels of phospholipid in the aorta did not increase with age but correlated with those of LDL cholesterol in plasma; levels of 17 beta-oestradiol did not change, but those of cGMP increased with age. 4. In endothelium-intact rings, the maximum tension developed by exposure to NA did not change, but the EC50 of NA increased with age and correlated with total cholesterol in the plasma and with the levels of all types of cholesterol in the aorta. In rings precontracted with NA, age decreased the maximum relaxation induced by ACh. The EC50 of ACh decreased with age and was inversely correlated with levels of cholesterol in the plasma and aorta. Treatment with NA increased cGMP levels in aged rats. Removal of the endothelium abolished the response to ACh and heightened the sensitivity to NA in young and aged rats. 5. Aortic endothelial cells seem to inhibit amine-induced contraction, while age-related changes in the levels of cholesterol in aortic tissue affect the sensitivity of the tissue to NA and ACh.