The exchange of unesterified cholesterol between plasma and red blood cells of young (1.5 month) and old (1-2 years) rats was studied using [3H]-cholesterol as label. Using labeled plasma and unlabeled red blood cells, no equilibration of specific activity of unesterified cholesterol with the specific activity of cholesterol of red blood cells occurred in either young or old rats. Using labeled red blood cells and unlabeled plasma, equilibration of radioactivity of cholesterol was observed in both young and old rats. However, a more rapid exchange of free cholesterol was observed when labeled old red blood cells were incubated with unlabeled plasma. This apparent paradox may be explained by the existence of a larger, more readily exchangeable pool of free cholesterol in red blood cells of old rats than of young rats. Red blood cells and plasma of old rats contained significantly more free cholesterol than those of young rats but phospholipid levels were comparable.